Question 1197736
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This is a supplement to @ikleyn's post.


The student asks "Why was 1 added to x/y?"


A better response would be to say "Because we factored out the M from {{{M + (x/y)*M}}} to end up with {{{M*(1+x/y)}}}"


The distributive rule is being applied
a*(b+c) = a*b+a*c
Factoring is effectively going in reverse of distributing.
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