x+y=7是x^2-y^2-6x+8y=7的()A,充要条件B,充分不必要条件C,必要不充分条件D,既请帮我详解

x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 【2(x-y)^3-8(x-y)^2(x+y)+6y(x-y)^2】/2(x-y)^2= 【2(x-y)(x-y)(x-y)-8(x-y)(x-y)(x+y)+6y(x-y)(x-y)]/2(x-y)(x-y) {6(x-y)-7(x+y)=21 {2(x-y)-5(x+y)=-1 若(x-y)/(x+y)=3 则(x+2y)/(6x-7y)等于 若x^2+y^2-6x+8y+25=0 则y/x-x/y的值是 已知X*X+Y*Y+6X-8Y+25=0 求（Y/X)+(X/Y)-2=? x*x+2x-y*y+6y-8=0,x+y不等于2,求x-y的值 已知：2x+y=6,x-3y=1 求7y(x-3y)(x-3y)-2(3y-x)(3y-x) 已知|5x-y-8|+(7y-x+2)=0则4x+6y 求几个方程组的答案 ｛2x+y=2 {3x-y=7 x=?y=?{x+2y=50 {3x=2y+6 x=?y=?{x+y=40 {10x+8y=370 x、y=?{x+y=5.8 {y=3x+0.6 x=?y=? 已知x、y满足方程组{7x+6(x-y)=130,则x--y的值是（ ）.{7y-2(x-y)=10 下列等式的右边是怎样从左边得到的?2x(x-y)^7/4y(y-x)^6=x(x-y)/2y “x+y=7”是“x^2-y^2-6x+8y=7”的?条件 3x+5y-(x+y)=6 2(x+y)-3x=-7 X+Y/X=2,X/Y=?是（x+y）/x=2哦 x+y=1 5x-2y=8 3x-y=7 2x-y=3x表示y和用y表示x 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