a²+b²
已是最简式不能再化简.若(x+y)²=23,(x-y)²=15,则xy的值为________，x²+y²的值为____________怎么做呢？(x+y)²=23X²+2xy+y²=23①(x-y)²=15x²-2xy+y²=15②①+②:2x²+2y²=38x²+y²=19①-②:4xy=8Xy=2