How to show gcd(ac, bd) = gcd(a,d) * gcd(b,c)

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Given that $gcd(a,b) = 1$ and $gcd(c,d) = 1$, show that $gcd(ac,bd) = gcd(a,d) * gcd(b,c)$



The work that I have done so far goes as follows.



We write $gcd(ac,bd) = acv + bdu$



then $gcd(a,d) = ax + dy$ and $gcd(b,c) = bs + ct$



then $gcd(a,d) * gcd(b,c) = (ax + dy) * (bs + ct) = axbs + axct + dybs + dyct$



I tried to factor out some terms, and get $ax(bs+ct) + dy(bs+ct)$
but then I am stuck here. I don't know what else to use to prove the answer










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    Please consider using MathJax to format your question - it will make your formulas much more readable
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up vote
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down vote

favorite
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Given that $gcd(a,b) = 1$ and $gcd(c,d) = 1$, show that $gcd(ac,bd) = gcd(a,d) * gcd(b,c)$



The work that I have done so far goes as follows.



We write $gcd(ac,bd) = acv + bdu$



then $gcd(a,d) = ax + dy$ and $gcd(b,c) = bs + ct$



then $gcd(a,d) * gcd(b,c) = (ax + dy) * (bs + ct) = axbs + axct + dybs + dyct$



I tried to factor out some terms, and get $ax(bs+ct) + dy(bs+ct)$
but then I am stuck here. I don't know what else to use to prove the answer










share|cite|improve this question









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Mario G is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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  • 1




    Please consider using MathJax to format your question - it will make your formulas much more readable
    – Zubin Mukerjee
    47 mins ago












up vote
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down vote

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up vote
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down vote

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Given that $gcd(a,b) = 1$ and $gcd(c,d) = 1$, show that $gcd(ac,bd) = gcd(a,d) * gcd(b,c)$



The work that I have done so far goes as follows.



We write $gcd(ac,bd) = acv + bdu$



then $gcd(a,d) = ax + dy$ and $gcd(b,c) = bs + ct$



then $gcd(a,d) * gcd(b,c) = (ax + dy) * (bs + ct) = axbs + axct + dybs + dyct$



I tried to factor out some terms, and get $ax(bs+ct) + dy(bs+ct)$
but then I am stuck here. I don't know what else to use to prove the answer










share|cite|improve this question









New contributor




Mario G is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











Given that $gcd(a,b) = 1$ and $gcd(c,d) = 1$, show that $gcd(ac,bd) = gcd(a,d) * gcd(b,c)$



The work that I have done so far goes as follows.



We write $gcd(ac,bd) = acv + bdu$



then $gcd(a,d) = ax + dy$ and $gcd(b,c) = bs + ct$



then $gcd(a,d) * gcd(b,c) = (ax + dy) * (bs + ct) = axbs + axct + dybs + dyct$



I tried to factor out some terms, and get $ax(bs+ct) + dy(bs+ct)$
but then I am stuck here. I don't know what else to use to prove the answer







elementary-number-theory






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Mario G is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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edited 44 mins ago









greedoid

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Mario G is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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Check out our Code of Conduct.







  • 1




    Please consider using MathJax to format your question - it will make your formulas much more readable
    – Zubin Mukerjee
    47 mins ago












  • 1




    Please consider using MathJax to format your question - it will make your formulas much more readable
    – Zubin Mukerjee
    47 mins ago







1




1




Please consider using MathJax to format your question - it will make your formulas much more readable
– Zubin Mukerjee
47 mins ago




Please consider using MathJax to format your question - it will make your formulas much more readable
– Zubin Mukerjee
47 mins ago










3 Answers
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Take all prime factors in turn. We have the following relations between the multiplicities:



$$gcd(a,b) = 1iffmin(alpha,beta)=0,$$
$$gcd(a,b) = 1iffmin(gamma,delta)=0,$$



$$gcd(ac,bd) = gcd(a,d) gcd(b,c)iffmin(alpha+gamma,beta+delta)=min(alpha,delta)+min(beta,gamma).$$



If we consider the case $alpha=gamma=0$,



$$min(0,beta+delta)=min(0,delta)+min(beta,0)$$ does hold.



Then with $beta=gamma=0$,



$$min(alpha,delta)=min(alpha,delta).$$
By symmetry other cases hold.






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    Let $x = gcd(a,d)$ and $y= gcd(b,c)$, then $xymid ac$ and $xymid bd$ so $boxedxymid z$ where $z=gcd(ac,bd)$.




    Vice versa:



    We can write $a=xa'$ and $d=xd'$ where $gcd(a',d')=1$ and $b=yb'$ and $c=yc'$ where $gcd(b',c')=1$



    Now since $zmid ac = xya'c'$ and $zmid bd = xyb'd'$



    Now if there is prime $p$ such that $pmid z$ and $gcd(xy,p)=1$ then $pmid a'c'$ and $pmid b'd'$. If $pmid a'$ then $pmid b'$ since $gcd(a',d')=1$. But then $pmid a$ and $pmid b$ so $pmid gcd(a,b)=1$ a contradiction. So there is no such $p$ and thus $boxedzmid xy$






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    • What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
      – Mario G
      30 mins ago










    • $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
      – greedoid
      28 mins ago










    • The greatest common divisor is a multiple of every other common divisor.
      – PossiblyDakota
      26 mins ago










    • You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
      – Hw Chu
      25 mins ago


















    up vote
    1
    down vote













    Let $gcd(a,d) = g$ so that $a = a'g$ and $d = d'g$ and $gcd(a',d') =1$. (why?)



    And let $gcd(b,c) = h$ so that $b =b'h$ and $c= c'h$ and $gcd(b',c') =1$ (why?).



    Then $gcd(ac,bd) = gcd(a'c'gh, b'd'gh) = gh*gcd(a'c',b'd')$. [$gcd(m*k, n*k) = kgcd(m,n)$. (why?)]



    And if you take any prime factor of $a'c'$ then it is a prime factor of $a'$ or $c'$ so it is not a prime factor of either $b'$ or $d'$ (as those both coprime to both $a'$ and $c'$; $a$ and $b$ are coprime, $c$ and $d$ are coprime, and so are $a'$ and $d'$ and so are $b'$ and $c'$). So it is not a prime factor of $b'd'$. Likewise no prime factor of $b'd'$ is a prime factor of $a'c'$. So $gcd(a'c', b'd') = 1$.



    So $gcd(ac, bd) = gh = gcd(a,d)gcd(b,c)$.






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      3 Answers
      3






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      3 Answers
      3






      active

      oldest

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      active

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      active

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      up vote
      3
      down vote













      Take all prime factors in turn. We have the following relations between the multiplicities:



      $$gcd(a,b) = 1iffmin(alpha,beta)=0,$$
      $$gcd(a,b) = 1iffmin(gamma,delta)=0,$$



      $$gcd(ac,bd) = gcd(a,d) gcd(b,c)iffmin(alpha+gamma,beta+delta)=min(alpha,delta)+min(beta,gamma).$$



      If we consider the case $alpha=gamma=0$,



      $$min(0,beta+delta)=min(0,delta)+min(beta,0)$$ does hold.



      Then with $beta=gamma=0$,



      $$min(alpha,delta)=min(alpha,delta).$$
      By symmetry other cases hold.






      share|cite|improve this answer


























        up vote
        3
        down vote













        Take all prime factors in turn. We have the following relations between the multiplicities:



        $$gcd(a,b) = 1iffmin(alpha,beta)=0,$$
        $$gcd(a,b) = 1iffmin(gamma,delta)=0,$$



        $$gcd(ac,bd) = gcd(a,d) gcd(b,c)iffmin(alpha+gamma,beta+delta)=min(alpha,delta)+min(beta,gamma).$$



        If we consider the case $alpha=gamma=0$,



        $$min(0,beta+delta)=min(0,delta)+min(beta,0)$$ does hold.



        Then with $beta=gamma=0$,



        $$min(alpha,delta)=min(alpha,delta).$$
        By symmetry other cases hold.






        share|cite|improve this answer
























          up vote
          3
          down vote










          up vote
          3
          down vote









          Take all prime factors in turn. We have the following relations between the multiplicities:



          $$gcd(a,b) = 1iffmin(alpha,beta)=0,$$
          $$gcd(a,b) = 1iffmin(gamma,delta)=0,$$



          $$gcd(ac,bd) = gcd(a,d) gcd(b,c)iffmin(alpha+gamma,beta+delta)=min(alpha,delta)+min(beta,gamma).$$



          If we consider the case $alpha=gamma=0$,



          $$min(0,beta+delta)=min(0,delta)+min(beta,0)$$ does hold.



          Then with $beta=gamma=0$,



          $$min(alpha,delta)=min(alpha,delta).$$
          By symmetry other cases hold.






          share|cite|improve this answer














          Take all prime factors in turn. We have the following relations between the multiplicities:



          $$gcd(a,b) = 1iffmin(alpha,beta)=0,$$
          $$gcd(a,b) = 1iffmin(gamma,delta)=0,$$



          $$gcd(ac,bd) = gcd(a,d) gcd(b,c)iffmin(alpha+gamma,beta+delta)=min(alpha,delta)+min(beta,gamma).$$



          If we consider the case $alpha=gamma=0$,



          $$min(0,beta+delta)=min(0,delta)+min(beta,0)$$ does hold.



          Then with $beta=gamma=0$,



          $$min(alpha,delta)=min(alpha,delta).$$
          By symmetry other cases hold.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited 8 mins ago

























          answered 26 mins ago









          Yves Daoust

          115k667210




          115k667210




















              up vote
              2
              down vote













              Let $x = gcd(a,d)$ and $y= gcd(b,c)$, then $xymid ac$ and $xymid bd$ so $boxedxymid z$ where $z=gcd(ac,bd)$.




              Vice versa:



              We can write $a=xa'$ and $d=xd'$ where $gcd(a',d')=1$ and $b=yb'$ and $c=yc'$ where $gcd(b',c')=1$



              Now since $zmid ac = xya'c'$ and $zmid bd = xyb'd'$



              Now if there is prime $p$ such that $pmid z$ and $gcd(xy,p)=1$ then $pmid a'c'$ and $pmid b'd'$. If $pmid a'$ then $pmid b'$ since $gcd(a',d')=1$. But then $pmid a$ and $pmid b$ so $pmid gcd(a,b)=1$ a contradiction. So there is no such $p$ and thus $boxedzmid xy$






              share|cite|improve this answer






















              • What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
                – Mario G
                30 mins ago










              • $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
                – greedoid
                28 mins ago










              • The greatest common divisor is a multiple of every other common divisor.
                – PossiblyDakota
                26 mins ago










              • You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
                – Hw Chu
                25 mins ago















              up vote
              2
              down vote













              Let $x = gcd(a,d)$ and $y= gcd(b,c)$, then $xymid ac$ and $xymid bd$ so $boxedxymid z$ where $z=gcd(ac,bd)$.




              Vice versa:



              We can write $a=xa'$ and $d=xd'$ where $gcd(a',d')=1$ and $b=yb'$ and $c=yc'$ where $gcd(b',c')=1$



              Now since $zmid ac = xya'c'$ and $zmid bd = xyb'd'$



              Now if there is prime $p$ such that $pmid z$ and $gcd(xy,p)=1$ then $pmid a'c'$ and $pmid b'd'$. If $pmid a'$ then $pmid b'$ since $gcd(a',d')=1$. But then $pmid a$ and $pmid b$ so $pmid gcd(a,b)=1$ a contradiction. So there is no such $p$ and thus $boxedzmid xy$






              share|cite|improve this answer






















              • What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
                – Mario G
                30 mins ago










              • $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
                – greedoid
                28 mins ago










              • The greatest common divisor is a multiple of every other common divisor.
                – PossiblyDakota
                26 mins ago










              • You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
                – Hw Chu
                25 mins ago













              up vote
              2
              down vote










              up vote
              2
              down vote









              Let $x = gcd(a,d)$ and $y= gcd(b,c)$, then $xymid ac$ and $xymid bd$ so $boxedxymid z$ where $z=gcd(ac,bd)$.




              Vice versa:



              We can write $a=xa'$ and $d=xd'$ where $gcd(a',d')=1$ and $b=yb'$ and $c=yc'$ where $gcd(b',c')=1$



              Now since $zmid ac = xya'c'$ and $zmid bd = xyb'd'$



              Now if there is prime $p$ such that $pmid z$ and $gcd(xy,p)=1$ then $pmid a'c'$ and $pmid b'd'$. If $pmid a'$ then $pmid b'$ since $gcd(a',d')=1$. But then $pmid a$ and $pmid b$ so $pmid gcd(a,b)=1$ a contradiction. So there is no such $p$ and thus $boxedzmid xy$






              share|cite|improve this answer














              Let $x = gcd(a,d)$ and $y= gcd(b,c)$, then $xymid ac$ and $xymid bd$ so $boxedxymid z$ where $z=gcd(ac,bd)$.




              Vice versa:



              We can write $a=xa'$ and $d=xd'$ where $gcd(a',d')=1$ and $b=yb'$ and $c=yc'$ where $gcd(b',c')=1$



              Now since $zmid ac = xya'c'$ and $zmid bd = xyb'd'$



              Now if there is prime $p$ such that $pmid z$ and $gcd(xy,p)=1$ then $pmid a'c'$ and $pmid b'd'$. If $pmid a'$ then $pmid b'$ since $gcd(a',d')=1$. But then $pmid a$ and $pmid b$ so $pmid gcd(a,b)=1$ a contradiction. So there is no such $p$ and thus $boxedzmid xy$







              share|cite|improve this answer














              share|cite|improve this answer



              share|cite|improve this answer








              edited 22 mins ago

























              answered 32 mins ago









              greedoid

              29.1k93879




              29.1k93879











              • What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
                – Mario G
                30 mins ago










              • $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
                – greedoid
                28 mins ago










              • The greatest common divisor is a multiple of every other common divisor.
                – PossiblyDakota
                26 mins ago










              • You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
                – Hw Chu
                25 mins ago

















              • What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
                – Mario G
                30 mins ago










              • $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
                – greedoid
                28 mins ago










              • The greatest common divisor is a multiple of every other common divisor.
                – PossiblyDakota
                26 mins ago










              • You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
                – Hw Chu
                25 mins ago
















              What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
              – Mario G
              30 mins ago




              What is the property you are using in the first line where $xy|ac$ and $xy|bd$ so $xy|z$ ?
              – Mario G
              30 mins ago












              $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
              – greedoid
              28 mins ago




              $xy$ divide $ac$ and $bd$ so it must divide the greates common divisor of them.
              – greedoid
              28 mins ago












              The greatest common divisor is a multiple of every other common divisor.
              – PossiblyDakota
              26 mins ago




              The greatest common divisor is a multiple of every other common divisor.
              – PossiblyDakota
              26 mins ago












              You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
              – Hw Chu
              25 mins ago





              You did not use the fact that $gcd(a,b) = gcd(c,d) = 1$. As your "counterexample" did not satisfy the assumption.
              – Hw Chu
              25 mins ago











              up vote
              1
              down vote













              Let $gcd(a,d) = g$ so that $a = a'g$ and $d = d'g$ and $gcd(a',d') =1$. (why?)



              And let $gcd(b,c) = h$ so that $b =b'h$ and $c= c'h$ and $gcd(b',c') =1$ (why?).



              Then $gcd(ac,bd) = gcd(a'c'gh, b'd'gh) = gh*gcd(a'c',b'd')$. [$gcd(m*k, n*k) = kgcd(m,n)$. (why?)]



              And if you take any prime factor of $a'c'$ then it is a prime factor of $a'$ or $c'$ so it is not a prime factor of either $b'$ or $d'$ (as those both coprime to both $a'$ and $c'$; $a$ and $b$ are coprime, $c$ and $d$ are coprime, and so are $a'$ and $d'$ and so are $b'$ and $c'$). So it is not a prime factor of $b'd'$. Likewise no prime factor of $b'd'$ is a prime factor of $a'c'$. So $gcd(a'c', b'd') = 1$.



              So $gcd(ac, bd) = gh = gcd(a,d)gcd(b,c)$.






              share|cite|improve this answer
























                up vote
                1
                down vote













                Let $gcd(a,d) = g$ so that $a = a'g$ and $d = d'g$ and $gcd(a',d') =1$. (why?)



                And let $gcd(b,c) = h$ so that $b =b'h$ and $c= c'h$ and $gcd(b',c') =1$ (why?).



                Then $gcd(ac,bd) = gcd(a'c'gh, b'd'gh) = gh*gcd(a'c',b'd')$. [$gcd(m*k, n*k) = kgcd(m,n)$. (why?)]



                And if you take any prime factor of $a'c'$ then it is a prime factor of $a'$ or $c'$ so it is not a prime factor of either $b'$ or $d'$ (as those both coprime to both $a'$ and $c'$; $a$ and $b$ are coprime, $c$ and $d$ are coprime, and so are $a'$ and $d'$ and so are $b'$ and $c'$). So it is not a prime factor of $b'd'$. Likewise no prime factor of $b'd'$ is a prime factor of $a'c'$. So $gcd(a'c', b'd') = 1$.



                So $gcd(ac, bd) = gh = gcd(a,d)gcd(b,c)$.






                share|cite|improve this answer






















                  up vote
                  1
                  down vote










                  up vote
                  1
                  down vote









                  Let $gcd(a,d) = g$ so that $a = a'g$ and $d = d'g$ and $gcd(a',d') =1$. (why?)



                  And let $gcd(b,c) = h$ so that $b =b'h$ and $c= c'h$ and $gcd(b',c') =1$ (why?).



                  Then $gcd(ac,bd) = gcd(a'c'gh, b'd'gh) = gh*gcd(a'c',b'd')$. [$gcd(m*k, n*k) = kgcd(m,n)$. (why?)]



                  And if you take any prime factor of $a'c'$ then it is a prime factor of $a'$ or $c'$ so it is not a prime factor of either $b'$ or $d'$ (as those both coprime to both $a'$ and $c'$; $a$ and $b$ are coprime, $c$ and $d$ are coprime, and so are $a'$ and $d'$ and so are $b'$ and $c'$). So it is not a prime factor of $b'd'$. Likewise no prime factor of $b'd'$ is a prime factor of $a'c'$. So $gcd(a'c', b'd') = 1$.



                  So $gcd(ac, bd) = gh = gcd(a,d)gcd(b,c)$.






                  share|cite|improve this answer












                  Let $gcd(a,d) = g$ so that $a = a'g$ and $d = d'g$ and $gcd(a',d') =1$. (why?)



                  And let $gcd(b,c) = h$ so that $b =b'h$ and $c= c'h$ and $gcd(b',c') =1$ (why?).



                  Then $gcd(ac,bd) = gcd(a'c'gh, b'd'gh) = gh*gcd(a'c',b'd')$. [$gcd(m*k, n*k) = kgcd(m,n)$. (why?)]



                  And if you take any prime factor of $a'c'$ then it is a prime factor of $a'$ or $c'$ so it is not a prime factor of either $b'$ or $d'$ (as those both coprime to both $a'$ and $c'$; $a$ and $b$ are coprime, $c$ and $d$ are coprime, and so are $a'$ and $d'$ and so are $b'$ and $c'$). So it is not a prime factor of $b'd'$. Likewise no prime factor of $b'd'$ is a prime factor of $a'c'$. So $gcd(a'c', b'd') = 1$.



                  So $gcd(ac, bd) = gh = gcd(a,d)gcd(b,c)$.







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                  answered 16 mins ago









                  fleablood

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