A 56 cm long wire is used to form an isosceles trapezoid, the sum of the two waist lengths is 36 cm, and the height is 7 cm. What is its area in square centimetres.
用一根长56厘米的铁丝围成一个等腰梯形，两条腰长之和是36厘米，高是7厘米．它的面积是多少平方厘米．
<gadget id="calculator">56 - 36</gadget>
<output>20</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">20 * (7/2)</gadget>
<output>70</output>

<result>70</result>