Evaluate
-\left(a+2b\right)^{2}+81
Expand
81-4b^{2}-4ab-a^{2}
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81-\left(a^{2}+4ab+4b^{2}\right)
Use binomial theorem \left(p+q\right)^{2}=p^{2}+2pq+q^{2} to expand \left(a+2b\right)^{2}.
81-a^{2}-4ab-4b^{2}
To find the opposite of a^{2}+4ab+4b^{2}, find the opposite of each term.
81-\left(a^{2}+4ab+4b^{2}\right)
Use binomial theorem \left(p+q\right)^{2}=p^{2}+2pq+q^{2} to expand \left(a+2b\right)^{2}.
81-a^{2}-4ab-4b^{2}
To find the opposite of a^{2}+4ab+4b^{2}, find the opposite of each term.
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