9^(n+1)-3^2n=72
    9*9^n-9^n=72
    8*9^n=72
    9^n=9=9^1
    n=1

20^n=(4*5)^n
        =4^n * 5^n
        =3*2
        =6

2^8 * 0.5^9 = 2^8 * (1/2)^9
                   =1/2

∵ (a+1)^2+|b-3|=0
∴ (a+1)^2=0
     |b-1|=0
∴ a=-1
     b=3
∴ a^2011 * b^3 = 26

(a+b)(a-b)(a+b)(a+b)=(a^2-b^2)(a^2+2ab+b^2)
                                 =a^4-b^4+2a^3b-2ab^3

a^(6n+3)
=[a^(2n+1)]^3
=5^3
=125

2^555=(2^5)^111=32^111
3^333=(3^3)^111=27^111
5^222=(5^2)^111=25^111
∵  25<27<32
∴  (25)^111<(27)^111<(32)^111
∴  5^222<3^333<2^555.