4x+(40-x)X2=100
4x+40X2-xX2=100
4x-2x+80=100
2x=100-80=20
x=10
4x+(40-x)X2=100
4x+40*2-2x=100
4x+80-2x=100
2x=100-80
2x=20
x=10
先去括号
4x+80-2x=100
合并同类项
2x=100-80
x=10
(X-30)[1500+400(40-X)]=17500
(x-30)(17500-400x)=17500
(x-30)(175-4x)=175
175x-4x^2-5250+120x=175
4x^2-295x+5425=0
(4x-155)(x-35)=0
x1=155/4
x2=35