Find \(N\),
the number of the turns, as a function of the other variables in the formula for the
magnetic induction of a solenoid.
\[
B =\mu \frac{NI}
{l}
\]
Find the focus length \(f\)
as a function of the other variables from the following equation relating this distance with object
and image distances \(a\)
and \(a'\).
\[
\frac{1}
{f} = \frac{1}
{a} + \frac{1}
{a'}
\]