The perimeter of an isosceles triangle is 15 cm, the waist length is 4 cm, and the height above the base is 9 cm. What is its area?
一个等腰三角形的周长是15厘米，腰长4厘米，底边上的高是9厘米，它的面积是多少．
<gadget id="calculator">15 - 4 - 4</gadget>
<output>7</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">9 * (7/2)</gadget>
<output>63/2 = around 31.5</output>

<result>63/2 = around 31.5</result>