Solve for a_n
a_{n}=\left(n+1\right)^{2}
Solve for n
n=-\left(\sqrt{a_{n}}+1\right)
n=\sqrt{a_{n}}-1\text{, }a_{n}\geq 0
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a_{n}=n^{2}+2n+1
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(n+1\right)^{2}.
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