已知X^2-2XY=47,3XY+4Y^2=-19,则3X^2+8Y^2=多少?
问题描述：
已知X^2-2XY=47,3XY+4Y^2=-19,则3X^2+8Y^2=多少?
答
X^2-2XY=47
3X^2-6XY=141 (1)
3XY+4Y^2=-19
6XY+8Y^2=-38 (2)
(1)+(2)得
3X^2+8Y^2=103