第7题看的不是很清楚：
题目如果是：9^n*27^(n-1)÷3^(3n+1)=81
即：3^[2n+3(n-1)-(3n+1)]=3^4
2n+3n-3-3n-3=4
所以2n=10
即n=5,则有n^(-2)=5^(-2)=1/25.
第八题：
1:
a^3b^3[2a^(-1)b]^3
=a^3b^3*2^3*a^(-3)b^3
=8a^(3-3)b^(3+3)
=8b^6
2:
a^(-2)b^2[-2a^2b^(-2)]^(-2)÷[a^(-4)b^2]
=a^(-2)b^2*(-2)^(-2)a^(-4)b^4÷[a^(-4)b^2]
=(-2)^(-2)*a^[(-2)+(-4)-(-4)]b^(2+4-2)
=(1/4)a(-2)b^4
=b^4/(4a^2).
3:
3a^2b*[-2ab^(-2)]^2÷[4a^(-2)b^(-3)]
=3a^2b*(-2)^2a^2b^(-4)÷[4a^(-2)b^(-3)]
=3*(-2)^2*a^[2+2-(-2)]b^[1+(-4)-(-3)]
=6a^6b^0
=6a^6
4:
[-2^(-1)mn^(-2)]^(-2)(m^2n)^(-3)÷2m^(-2)
=(-1)^(-2)*2^[(-1)*(-2)]*m^(-2)*n^[(-2)*(-2)]*m^(-6)n^(-3)÷2m^(-2)
=2^(2-1)*m^[(-2)+(-6)-(-2)]n^[4+(-3)]
=2m^(-6)n
=2n/m^6.