思路:
0.5(x+y+z)方+0.5(x-y-z)(x-y+z)-2(x+y)
=0.5[(x+y)方+2z(x+y)+z方]+0.5[(x-y)方-z方]-z(x+y)
=0.5(x+y)方+0.5(x-y)方=x方＋y方
由x-y=6,xy=21得,
x方＋y方＝（x-y）方＋2xy＝78
注:y方是y的平方.