16(x-y)的4次方-72(x-y)的平方+81
=16(x-y)^4-72(x-y)^2+81
=[4(x-y)^2-9]^2
={[2(x-y)+3][2(x-y)-3]}^2
(x的平方-4x）的平方+8（x的平方-4x)+16
=(x^2-4x)^2+8(x^2-4x)+16
=[(x^2-4x)+4]^2
=(x^2-4x+4)^2
=[(x-2)^2]^2
=(x-2)^4
-a的平方b的平方+2abc的平方-c的4次方
=-a^2b^2+2abc^2-c^4
=-(ab)^2+2abc^2-c^4
=-[(ab)^2-2abc+c^4]
=-(ab-c^2)^2
9分之m的平方+n的平方-2/3mn
=m^2/9+n^2-2/3mn
=(m/3-n)^2