2x+3y=15 5x-7y=23

已知x-y/x+y=3,求代数式5(x-y)/x+y-x+y/2(x-y) 先化简再求值(x-y)(x+y)-(x-2y) 的完全平方+x(3x-5y)-(x-y)(x-2y),其中x=1/2 y 解方程组(x+y)(x-2y)=-5 (x-y)(x+2y)=7 6(x-y)-2(x+y)=14 3(x-y)+(x+y)=5 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 若(x-y)/(x+y)=3 则(x+2y)/(6x-7y)等于 x+y=3 x+2y=9 3/x+1=y y-x=3 y=2x-3 y-3x=1 2(x+1)-y=6 7x-5y=-9 {2x+y/3-2y=0 2(2x+y)-5=7y 7x-3y/5=5x-y/3-x+y/2 x-1/5=y+1/3 解方程x+y+(x+y/2)=15 3x+5y+4(x+y/2)=62 解二元一次方程组:{5(x+y)+3(x-y)=2{7(x+y)-3(x-y)=-6 10/x+y+3/x-y=-5 15/x+y-2/x-y=-1（用换元法解此方程组）