a) \(9-\left(x+y\right)^2=3^2-\left(x+y\right)^2\)
\(=\left[3-\left(x+y\right)\right]\left[3+\left(x+y\right)\right]\)
\(=\left(3-x-y\right)\left(3+x+y\right)\)
b)\(x^4-1=\left(x^2\right)^2-1^2\)
\(=\left(x^2-1\right)\left(x^2+1\right)\)
\(=\left(x-1\right)\left(x+1\right)\left(x^2+1\right)\)
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