[语言过关]求梯形的面积

<h3 style="text-align:center;font-family:'Microsoft Yahei';background-color:#F5F5F5;"> <span style="font-size:18px;">求梯形的面积（</span><span style="font-size:18px;">tx.pas/c/cpp）</span> </h3> <p style="font-family:'Microsoft Yahei';font-size:14px;background-color:#F5F5F5;"> <span style="font-size:18px;">&nbsp; &nbsp; 已知梯形的上底为a厘米，下底为b厘米，高位h厘米，求梯形的面积。</span> </p> <h3 style="font-family:'Microsoft Yahei';background-color:#F5F5F5;"> <span style="font-size:18px;">输入格式</span> </h3> <p style="font-family:'Microsoft Yahei';font-size:14px;background-color:#F5F5F5;"> <span style="font-size:18px;">输入数据为一行3个实数，a，b，h分别为梯形的上底、下底和高。</span> </p> <h3 style="font-family:'Microsoft Yahei';background-color:#F5F5F5;"> <span style="font-size:18px;">输出</span> </h3> <p style="font-family:'Microsoft Yahei';font-size:14px;background-color:#F5F5F5;"> <span style="font-size:18px;">只有一行，输出梯形的面积。输出的结果保留两位小数。</span> </p> <h3 style="font-family:'Microsoft Yahei';background-color:#F5F5F5;"> <span style="font-size:18px;">样例输入</span> </h3> <p style="font-family:'Microsoft Yahei';font-size:14px;background-color:#F5F5F5;"> <span style="font-family:'Lucida Console';font-size:18px;">15&nbsp;25&nbsp;20</span> </p> <h3 style="font-family:'Microsoft Yahei';background-color:#F5F5F5;"> <span style="font-size:18px;">样例输出</span> </h3> <p style="font-family:'Microsoft Yahei';font-size:14px;background-color:#F5F5F5;"> <span style="font-family:'Lucida Console';font-size:18px;">400.00</span> </p>