Evaluate
\left(a+2b\right)^{2}-9b
Expand
a^{2}+4ab+4b^{2}-9b
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\left(2b+a\right)^{2}-9b
Combine 3b and -b to get 2b.
4b^{2}+4ba+a^{2}-9b
Use binomial theorem \left(p+q\right)^{2}=p^{2}+2pq+q^{2} to expand \left(2b+a\right)^{2}.
\left(2b+a\right)^{2}-9b
Combine 3b and -b to get 2b.
4b^{2}+4ba+a^{2}-9b
Use binomial theorem \left(p+q\right)^{2}=p^{2}+2pq+q^{2} to expand \left(2b+a\right)^{2}.
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