99^2+198+1 =99^2+2x1x99+1 =（99+1)²=100x100=10000（x+y-z)^2=（x+y-z)（x+y-z)= x²+xy-xz+xy+y²-yz-xz-yz+z²= x²+2xy-2xz-2yz+y²+z²