After the cylinder is cut into two half cylinders from the diameter along the height, the surface area increases by 40 square centimeters, find the lateral area of the cylinder.
把圆柱从直径沿着高切成两个半圆柱后，表面积增加了40平方厘米，求圆柱侧面积．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">3.14 * 20</gadget>
<output>62.8</output>

<result>62.8</result>