Answer
$f_2=15.8Hz$
Work Step by Step
We will use the following formula here:
$f_n=\frac{n}{2}\sqrt\frac{T}{LM}$
We plug in the known values to obtain:
$f_n=\frac{n}{2}\sqrt\frac{250}{(10)(0.1)}=n(7.91)Hz$
Therefore, the second lowest frequency is:
$f_2=2(7.91)=15.8Hz$
