produsul mezilor egal cu produsul extremilor
(2x+3)(9-4x²)=(2x-3)(-4x²-12x-9)
(2x+3)((3-2x)(3+2x)=-(2x-3)(2x+3)²
-(2x+3)²(2x-3)+(2x-3)(2x+3)²=0, 0=0, x∈R
x(x³-x²+x-1)=(x-1)(x³+x)
x[x²(x-1)+(x-1)]=x(x-1)(x²+1)
x(x-1)(x²+1)=x(x-1)(x²+1), 0=0, x∈R