Question

如果x的平方+2x=3,求x的四次方+7x的三次方+8x的平方-13x+15的值.

Answer 1

x^4+7x^3+8x^2-13x+15

=x^4+2x^3+5x^3+10x^2-2x^2-13x+15

=x^2(x^2+2x)+5x(x^2+2x)-2x^2-13x+15

=3x^2+15x-2x^2-13x+15

=x^2+2x+15

=3+15

=18