13:
\(9\left(x-y\right)^2-4\left(x+y\right)^2\)
\(=\left(3x-3y\right)^2-\left(2x+2y\right)^2\)
\(=\left(3x-3y-2x-2y\right)\left(3x-3y+2x+2y\right)\)
\(=\left(x-5y\right)\left(5x-y\right)\)
14: \(\left(3x-2y\right)^2-\left(2x-3y\right)^2\)
\(=\left(3x-2y-2x+3y\right)\left(3x-2y+2x-3y\right)\)
\(=\left(x+y\right)\left(5x-5y\right)\)
\(=5\left(x-y\right)\left(x+y\right)\)
15: \(4x^2-4x+1-\left(x+1\right)^2\)
\(=\left(2x-1\right)^2-\left(x+1\right)^2\)
\(=\left(2x-1-x-1\right)\left(2x-1+x+1\right)\)
\(=3x\cdot\left(x-2\right)\)
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