Question

1.X+Y=5,X-Y=－2,则X²+Y²的值是多少?

Answer 1

X²+Y²=(X+Y)²-2XY=25-2XY (1)

X²+Y²=(X-Y)²+2XY=4+2XY (2)

(1)+(2)

2(X²+Y²)=25+4

X²+Y²=14.5