The perimeter of an isosceles right triangle is 21 cm and the length of the hypotenuse is 9 cm. What is the area of this isosceles triangle?
一等腰直角三角形的周长为21厘米，斜边长9厘米，这个等腰三角形的面积是多少．
<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<result>18</result>