4x-3y=18 3x-4y=17 x/2+y/3=2 0.2x+4x-3y=18 3x-4y=17x/2+y/3=2 0.2x+0.3y=2.82x+y/3=3x-2y/8=3x+y+z=20 2x+y+z=42 x-5z=8错了。是x+y-z=20 2x+y+z=42 x-5z=8

(x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)= （3x-y)(y+3x)-(4x-3y)（4x+3y)=? {3(x+y)+3(y+x)=1,3(x+y)+4(y-x)=-1 2（x+y)-3(x-y)=3 ,4(x+y)+3(x-y)=15 （5x+3y)(3y-5x)-(4x-y)(4y+x)= {3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1 x+y/2+x-y/3=6,4(x+y)-3(x-y)=-20 {(x+y)/2+(x-y)/3=6 4(x+y)-3(x-y)=-20 4(x+y)-3(x-y)=-20,2/x+y+3/x-y=6 3(x+y)-2(x-y)=11 4(x+y)+3(x-y)=9 x(x-3y)=4y(x-3y),求x:y 4(x-y)-3(x-y)=9 5(x-y)+2(x-y)=11 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 3(x+y)+2(x-y)=36 4(x+y)-5(x-y)=2 （x+y）/2- x-y/2=6 3(x+y)=4(x-y) x+y/3+x-y/2=6 4(x+y)-2(x-y)=40 x/3=y/4,求(x*x-xy)/(x*x+y*y)的值 先化简再求值(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y) x=4 y=5