18.62 After cutting off one corner of a rectangle, a pentagon with side lengths 13, 19, 20, 25, and 31 (not necessarily in this order) is obtained. The area of the pentagon is 
(A) 459.
(B) 600.
(C) 680.
(D) 720.
(E) 745.
(46th American High School Mathematics Examination, 1995)
［Solution］Considering the three sides relationship of the cut right-angled triangle:
$$
(31-19)^{2}+(25-20)^{2}=13^{2} \text {, }
$$

we know the original rectangle has side lengths of 25 and 31 (as shown in the right figure).
$$
\begin{aligned}
\therefore S_{\text {pentagon }} & =S_{\text {rectangle }}-S_{\text {right-angled triangle }} \\
& =31 \times 25-\frac{1}{2} \times 12 \times 5 \\
& =745 .
\end{aligned}
$$

Therefore, the answer is $(E)$.