就1道数学题,帮个忙啦已知X^2+Y^2=7,XY=2,求5X^2 -3XY -4Y^2 -11XY -7X^2 +2Y^2的值

问题描述:

就1道数学题,帮个忙啦
已知X^2+Y^2=7,XY=2,求5X^2 -3XY -4Y^2 -11XY -7X^2 +2Y^2的值

5X^2-3XY-4Y^2-11XY-7X^2+2Y^2
= X²-3XY-4Y²-11XY-7X²+2Y ²
=-2Y²-14XY-2X ²
=-2(x²+y²)-14xy
=-2*7-14*2
= -42

5X^2 -3XY -4Y^2 -11XY -7X^2 +2Y^2
=-2x^2-14xy-2y^2
=-2(x^2+y^2)-14xy
=-2*7-14*2
=-42