[问答题]
A diverging lens is placed n-lt k+gzj fgz h:ew2prpmg59-9jlr4 9 -t* mubext to a converginhzod;s1i+q q q9c8b2 +s fxtd+g lens of focal length $f_C$ as in (Fig. Below). If $f_T$ represents the focal length of the combination, show that the focal length of the diverging lens, $f_D$, is given by
$\frac{1}{f_\mathrm{D}}=\frac{1}{f_\mathrm{T}}-\frac{1}{f_\mathrm{C}}.$
