已知x²-y²=20,x+y=－5求y-x的值

已知x²-y²=20,x+y=－5求y-x的值
已知x²-y²=20,x+y=－5求y-x的值

已知x²-y²=20,x+y=－5求y-x的值
x²-y²=（x+y)*(x-y)=-5*(x-y)=20

所以：x-y=-4;
y-x=4

x^2-y^2=(x+y)(x-y)=20
又x+y=-5
所以x-y=-4
所以y-x=4

(X+Y)(X-Y)=20
X+Y=-5
两等式两边相除得：
X-Y=-4
Y-X=4

x^2-y^2=(x+y)(x-y)=20
x+y=-5
x-y=-4
y-x=4

x²-y²=(x+y)(x-y)=20,把x+y=－5代入，消去，的x-y=-4，所以y-x=4