#1
Here
[tex]\\ \tt\hookrightarrow log_57=\dfrac{log_{10}7}{log_{10}10}[/tex]
[tex]\\ \tt\hookrightarrow \dfrac{log_{10}7}{1}[/tex]
[tex]\\ \tt\hookrightarrow log_{10}7[/tex]
#2
[tex]\\ \tt\hookrightarrow log_38=\dfrac{log_e8}{log_ee}[/tex]
[tex]\\ \tt\hookrightarrow \dfrac{log_e8}{1}[/tex]
[tex]\\ \tt\hookrightarrow log_e8[/tex]