The upper base of a trapezoid is 9 cm, the lower base is 3 cm shorter, and the height is 10 cm. What is its area in square centimeters?
一个梯形的上底是9厘米，比下底短3厘米，高是10厘米，它的面积是多少平方厘米？
<gadget id="calculator">9 + 3 + 9</gadget>
<output>21</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">21 * 5</gadget>
<output>105</output>

<result>105</result>