a: \(=\dfrac{-\left(x-7\right)\left(x+7\right)\cdot3}{\left(x-7\right)\left(2x+1\right)}=\dfrac{-3x-21}{2x+1}\)
b: \(=\dfrac{\left(x-1\right)\left(x^2+x+1\right)}{2\left(x+2\right)}\cdot\dfrac{x^2+x+1-x^2+1}{\left(x-1\right)\left(x^2+x+1\right)}=\dfrac{1}{2}\)
c: \(=\left(\dfrac{4}{y\left(y-2\right)\left(y+2\right)}+\dfrac{1}{y+2}\right):\left(\dfrac{y-2}{y\left(y+2\right)}-\dfrac{y}{2\left(y+2\right)}\right)\)
\(=\dfrac{4+y\left(y-2\right)}{y\left(y-2\right)\left(y+2\right)}:\dfrac{2y-4-y^2}{y\left(y+2\right)\left(y-2\right)}\)
=-1
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