Is Divisibility Sufficient for Proving a Product Divides Another Product?

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Homework Statement


Show that if a | c and b | c, and (a, b) = d, then ab | cd.

Homework Equations


The Attempt at a Solution


Abstract divisibility.
We have c=am, c=bn, and d=an+bm.
 
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Could I multiply d=an+bm by c.
Then I have cd=acn+bcm
We have cd=c(an+bm)
cd=cd
Thus, ab | cd
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
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