问:已知x-y=3,x+y=7.求:x(x-y)-y(y-x)的值

问题描述:

问:已知x-y=3,x+y=7.求:x(x-y)-y(y-x)的值

x(x-y)-y(y-x)
=x(x-y)+y(x-y)
=(x-y)(x+y)
=3×7
=21