若x2-y2=20,且x+y=-5,则x-y的值是_.
问题描述:
若x2-y2=20,且x+y=-5,则x-y的值是______.
答
x2-y2=(x+y)(x-y)=-5(x-y)=20,
解得,x-y=-4.
故填:-4.