I am relatively new to proofs and can't seem to figure out how to solve an exercise.
I am trying to prove:
Suppose that $m$ and $n$ are perfect squares. Then $m+n+2\sqrt{mn}$ is also a perfect square.
I know that per the definition of a perfect square, that $m=a^2$ and $n=b^2$, if a and b are some positive integer.
I can then use substitution to rewrite the statement as:
$$a^2+b^2+2\sqrt{a^2b^2}$$
I also know that $2\sqrt{a^2b^2}$ can be simplified to:
S
$$a^2+b^2+2ab$$
I am stuck after this point though. I don't know how to eliminate the $2ab$.
Best Answer
You don't need to eliminate the $2ab$ term.
Notice that $(a+b)^2 = (a+b)(a+b) = a^2+ab+ba+b^2 = a^2+b^2+2ab$.