Question

已知x^2+3x-5=0,求2x^3+7x^2-7x+5的值

Answer 1

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

= 5 + 5 = 10