x+y=9, x-y=3 则2x^2- 2y^2的值是多少?
问题描述:
x+y=9, x-y=3 则2x^2- 2y^2的值是多少?
要过程.
答
2x^2- 2y^2
=2(x²-y²)
=2(x+y)(x-y)
=2×9×3
=54