(x-2)(x-5)(x-6)(x-15)+8x^2=（x^2+30-17x)(x^2+30-11x)+8x^2=(x^2+30-14x-3x)(x^2+30-14x+3x)+8x^2=(x^2+30-14x)^2-9x^2+8x^2=(x^2+30+x-14x)(x^2+30-x-14x)=(x-10)(x-3)(x^2+30-15x)