[(1/2)*a^(m+1)*b^(n-1)]^3=(1/8)*a^9*b^12
1/8*a^(3m+3)*b^(3n-3)=(1/8)*a^9*b^12
所以：
a^(3m+3)=a^9 3m+3=9 m=2
b^(3n-3)=b^12 3n-3=12 n=5