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$(a + b)(c + d) = a(c + d) + b(c + d)

= ac + ad + bc + bd$

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$(a+b)^2=a^2+2ab+b^2\\ (a-b)^2=a^2-2ab+b^2$

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$a^2-b^2=(a+b)(a+b)$

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Examples

Problem 1

Problem 2

Problem 3