{x+y=90,20%x+30%y=27%*90

{x+y=90,20%x+30%y=27%*90 5（x+y)+3(x-y)=90 5(x+y)-3(x-y)=30,解二元一次方程组 x+y=20-6 3x+y=30 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 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 |x|+|y|=30 2|x|-3|y|=20 求x,y （x+y)+(2x+1/1*2y)+(3x+1/2*3y)+.+(9x+1/8*9y) x=2 y=90求和 二元函数f(x,y)=x+y/x-y,求f(y/x,x/y) 有这样一道题,计算：（x-y)[(x+y)(x+y)-xy]-(x-y)[(x-y)(x-y)+xy]的值,其中x=20 [x(x-y)-y(x-y)+(x+y)(x-y)]÷2x其中x=2012 y=2013