Evaluate
\left(x+3\right)\left(x+15\right)
Expand
x^{2}+18x+45
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x^{2}+18x+81-36
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(x+9\right)^{2}.
x^{2}+18x+45
Subtract 36 from 81 to get 45.
x^{2}+18x+81-36
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(x+9\right)^{2}.
x^{2}+18x+45
Subtract 36 from 81 to get 45.
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