The lengths of the three sides of a triangle are 5cm, 7cm, and 9cm respectively. What is the perimeter of the triangle in cm?
三角形三条边的长度分别是5cm，7cm，9cm，这个三角形的周长是多少cm．
<gadget id="calculator">5 + 7 + 9</gadget>
<output>21</output>

<result>21</result>