x2-y2=x2-xy+xy-y2=5-3=2x2-2xy+y2=x2-xy-（xy-y2）=5-（-3）=8
x2-y2=x²-xy+xy-y²=5-3=2x2-2xy+y2=x²-xy-xy+y²=x²-xy-(xy-y²)=5+3=8
x2-y2=2x2-2xy+y2=8
x2-y2=2 x2-2xy+y2=8
