已知x^2+3x-5=0,求2x^3+7x^2-7x+5的值

问题描述:

已知x^2+3x-5=0,求2x^3+7x^2-7x+5的值

因为: x^2 + 3x - 5 = 0 所以, x^2 + 3x = 5因此, 2x^3 + 7x^2 - 7x + 5 =2x^3 + 6x^2 + x^2 - 7x + 5 = 2x(x^2 + 3x) + x^2- 7x +5 = 10x + x^2 - 7x + 5= x^2 + 3x + 5 ...