If the length of a rectangle is 12 cm and the width is 9 cm, what is the half of its perimeter in cm?
一个长方形的长是12厘米，宽是9厘米，它的周长的一半是多少厘米．
<gadget id="calculator">12 + 9</gadget>
<output>21</output>

<result>21</result>