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**Algebra Proofs!!**

I have two questions just to help verify what I'm doing:

Let R be a commutative ring with identity. Prove that R is an integral domain if and only if cancellation holds in R (that is, a no equal to 0 and ab=ac in R imply b=c)

=> Suppose cancellation holds: ab=0 -> ab=0a -> a isn't 0 so b=0

<= Since a doesn't equal 0

ab=ac -> ab-ac=0 -> a(b-c)=0 -> b-c=0 -> b=c cancellation holds

alternatively, could the second part be proven as such:

ab=ac

a^-1ab=aca^-1

b=c

cancellation holds

My second question is: How do I go about proving the following

(a^m)(a^n)=a^m+n and (a^m)^n=a^mn