a, 5\(^{-1}\)25\(^n\)=125
\(\frac{1}{5}\)25\(^n\)=125
25\(^n\)=625
n=2
b, 3\(^{-1}\)3\(^n\)+6. 3\(^{n-1}\)=7.3\(^6\)
3\(^{n-1}\)+6. 3\(^{n-1}\)=7.3\(^6\)
3\(^{n-1}\)(6+1)=7.3\(^6\)=3\(^{n-1}\).7
3\(^6\)=3\(^{n-1}\)
6=n-1
n=7
c,3\(^4\)<1/9.27^n <3^10
3^4<1/(3^2) .3^(3n)<3^10
3^4<3^(3n-2)<3^10
3n-2 chia 3 dư 1 nên 3n-2 = 7
n=3
d,25<5^n:5<625
5^2<5^(n-1)<5^4
n-1=3 nên n=4