The lateral expansion of a cylinder is a square with a side length of 21 cm. What is the lateral area of this cylinder in cm**2.
一个圆柱体的侧面展开是一个边长21cm的正方形．这个圆柱的侧面积是多少cm**2．
<gadget id="calculator">21 * 21</gadget>
<output>441</output>

<result>441</result>