已知(x+y)²=21,(x-y)²=15,求(1)x²+y²的值 (2)xy的值

问题描述:

已知(x+y)²=21,(x-y)²=15,求(1)x²+y²的值 (2)xy的值

⑴(x+y)²=21,(x-y)²=15,∵(x+y)²+(x-y)²=x²+2xy+y²+x²-2xy+y²=2x²+2y²∴2x²+2y²=21+15=36x²+y²=18⑵∵(x+y)²-(x-y)²...