CMR nếu \(\frac{x}{a}=\frac{y}{b}=\frac{z}{c}\)thì \(\left(x^2+y^2+z^2\right).\left(a^2+b^2+c^2\right)=\left(ax+bx+cz\right)^2\)
Cho đa thứ \(f\left(x\right)=x\left(x+1\right)\left(x+2\right)\left(ax+b\right)\)xác định a, b để \(f\left(x\right)-f\left(x-1\right)=x\left(x+1\right)\left(2x+1\right)\)\(\forall x\)