Evaluate
4\left(x-2y\right)\left(x+y\right)
Expand
4x^{2}-4xy-8y^{2}
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4x^{2}-4xy+y^{2}-9y^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(2x-y\right)^{2}.
4x^{2}-4xy-8y^{2}
Combine y^{2} and -9y^{2} to get -8y^{2}.
4x^{2}-4xy+y^{2}-9y^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(2x-y\right)^{2}.
4x^{2}-4xy-8y^{2}
Combine y^{2} and -9y^{2} to get -8y^{2}.
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