Question

已知x²+y²=7,xy= -2. 求5x²-3xy-4y²-11xy-7x²+2y²的值.

Answer 1

解：原式= 5x²-7x²-3xy-11xy -4y²+2y²

          = -2x²-14xy-2y²

          = -2(x²+y²)-14xy，

当x²+y²=7,xy= -2时，原式= -2×7-14×(-2) = -14+28 = 14.