求1/2x^2-1/4x+0.2x^3+0.25x-0.5x^2-1/5x^3+0.5x-1 其中x=12/13
问题描述：
求1/2x^2-1/4x+0.2x^3+0.25x-0.5x^2-1/5x^3+0.5x-1 其中x=12/13
答
1/2x^2-1/4x+0.2x^3+0.25x-0.5x^2-1/5x^3+0.5x-1
=(1/2x^2-0.5x^2)+(0.25x-1/4x)+(0.2x^3-1/5x^3)+0.5x-1
=0.5x-1
=0.5*12/13-13/13
=-7/13再细点1/2x^2-1/4x+0.2x^3+0.25x-0.5x^2-1/5x^3+0.5x-1 =1/2x^2-0.5x^2+0.25x-1/4x+0.2x^3-1/5x^3+0.5x-1（把X次方相同的项放在一起，调整相关数及其符号的位置）=(1/2x^2-0.5x^2)+(0.25x-1/4x)+(0.2x^3-1/5x^3)+0.5x-1（全是加减法，两两合并计算）=(0.5x^2-0.5x^2)+(0.25x-0.25x)+(0.2x^3-0.2x^3)+0.5x-1（1/2=0.5；1/4=0.25；1/5=0.2）=0+0+0+0.5x-1=0.5*12/13-13/13=-7/13