Answer
(a) $-2.75m$
(b) $-2.77m$
Work Step by Step
(a) We know that
$f=(\frac{1}{d_{\circ}}-\frac{1}{d_i})^{-1}$
We plug in the known values to obtain:
$f=(\frac{1}{25-0.0200}+\frac{1}{-(2.50-0.0200)})^{-1}=-2.75m$
(b) As $f=(\frac{1}{d_{\circ}}-\frac{1}{d_i})^{-1}$
We plug in the known values to obtain:
$f=(\frac{1}{25-0.0100}+\frac{1}{-(2.50-0.0100)})^{-1}=-2.77m$
