Use a square iron sheet with a side length of 5 decimeters to form a cylindrical ventilation pipe (the joints are not counted). What is the maximum lateral area of this vent pipe?
用边长5分米的正方形铁皮围成一个圆柱形通风管(接头处不计)．这个通风管的侧面积最大是多少？
<gadget id="calculator">5 * 5</gadget>
<output>25</output>

<result>25</result>