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Homework Help: GCD relative prime question

  1. Feb 20, 2015 #1
    1. The problem statement, all variables and given/known data
    if gcd(a,b) = 1, show that gcd(ac,b) = gcd(c,b)

    2. Relevant equations
    gcd(x,y) = xm + yn for integers n and m

    3. The attempt at a solution
    ax + by = 1
    gcd(ac,b) = d
    gcd(c,b) = g

    ac = dr
    b = ds
    c = gm
    b = gn

    I've been setting up equations and rearranging things but can't make any leeway, any tips?

    g|c and g|b, so g|ac and hence g|d.

    d|ac and d|b. If I can show that d|c then i can conclude d|g. hence d=g. I now need help showing that d|c.
    Last edited: Feb 20, 2015
  2. jcsd
  3. Feb 20, 2015 #2


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    Homework Helper

    If d is gcd(ac,b) then d|ac and d|b. If d >1, then d cannot divide both a and b, so d must divide c.
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