7x+2y=7,2x+7y=11求x+y有什么简便方法算我没用简便方法算的是 2 对了请写出简便方法解答过程

x+2y=2x+y+1=7x-y 求：2x-y? x^2+y^2=15,x+y=7,x＞y,求x-y 已知：2x+y=6,x-3y=1 求7y(x-3y)(x-3y)-2(3y-x)(3y-x) 7x+3y=44,11x+2y=61,求x,y 已知（x+y）2 =2,(x-y)3 =3,求(x+y)7(x-y)2(x+y)3(y-x)4 的值 已知实数x.y满足根号(x+y-8)+根号(8-x-y)=根号(3x-y-4)+根号(x-2y+7),求x,y {x+2y+2=0 {7y-4y=-41 求X和Y 若x-y=2(x+y),求代数式x-y/x+y+x+y/x-y求代数式 x+y分之x-y+x-y分之x+y 已知:y=ax的3次方+bx+3,当x=-1/2时,y=-7,试求x=1/2时,y的值 解方程组(x+y)(x-2y)=-5 (x-y)(x+2y)=7 {6(x-y)-7(x+y)=21 {2(x-y)-5(x+y)=-1 当x+y=2时7(x+y)一(x+y)+(x+y)=? 若x、y满足2x+y=6,x-3y=1,求7y(x-3y)²-2(3y-x)³的值 若(x-y)/(x+y)=3 则(x+2y)/(6x-7y)等于 求f(x,y)=x+y+7的最大值,使(x,y)满足x+2y≤10,2x+y≥6,y≥0 x+y=2,x^3+y^3=14,求x^4+y^4,x^7+y^7 已知x^2+y^2=25,x+y=7,且x小于y求x-y的值 方程组2x+y=7,x+2y=8.求x^2-y^2 x+2y+2=0 7x-4y=-41 求x和y