What is the base angle of an isosceles triangle whose apex angle is 120 degrees?
顶角是120度的等腰三角形的一个底角是多少．
<gadget id="calculator">180 - 120</gadget>
<output>60</output>

<gadget id="calculator">60 / 2</gadget>
<output>30</output>

<result>30</result>