\(=2\left(x^2+x-5\right)^2-5\left(x^2+x-5\right)+3\)
\(=2\left(x^2+x-5\right)-2\left(x^2+x-5\right)-3\left(x^2+x-5\right)+3\)
\(=2\left(x^2+x-5\right)\left(x^2+x-6\right)-3\left(x^2+x-6\right)\)
\(=\left(x^2+x-6\right)\left(2x^2+2x-13\right)\)
\(=\left(x-2\right)\left(x+3\right)\left(2x^2+2x-13\right)\)
\(C=2\left(x^2+x-5\right)^2-5\left(x^2+x\right)+28\)
Đặt t=\(x^2+x\)
\(\Rightarrow C=2\left(t-5\right)^2-5t+28=2t^2-20t+50-5t+28=2t^2-25t+78=2\left(t-\dfrac{13}{2}\right)\left(t-6\right)\)
Thay t: \(C=2\left(t-\dfrac{13}{2}\right)\left(t-6\right)=2\left(x^2+x-\dfrac{13}{2}\right)\left(x^2+x-6\right)=2\left(x-2\right)\left(x+3\right)\left(x^2+x-\dfrac{13}{2}\right)\)
Ta có: \(C=2\left(x^2+x-5\right)^2-5\left(x^2+x\right)+28\)
\(=2\left(x^2+x-5\right)^2-5\left(x^2+x-5\right)+3\)
\(=\left(x-2\right)\left(x+3\right)\left(2x^2+2x-13\right)\)