a, \(D=4x\left(x+y\right)-5y\left(x-y\right)-4x^2\)
\(=4x^2+4xy-5xy+5y^2-4x^2\)
\(=-xy+5y^2\)
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b, \(E=\left(a-1\right)\left(x^2+1\right)-x\left(y+1\right)+\left(x+y^2-a+1\right)\)
\(=ax^2+a-x^2-1-xy-x+x+y^2-a+1\)
\(=ax^2-x^2-xy+y^2\)
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