a²x²+16ax+64=(ax+8)²
a^4-8a²b²+16b^4=（a²-4b²)²=(a+2b)²(a-2b)²
（a+y）²-14（a+y）+49=[(a+y)-7]²=(a+y-7)²
x²y-2xy²+y³=y(x²-2xy+y²)=y(x-y)²
9x²-（2y+z）²=[3x+(2y+z)][3x-(2y+z)]=(3x+2y+z)(3x-2y-z)