If one angle of an isosceles triangle is 80°, what is its vertex angle?
等腰三角形的一个角是80°，则它的顶角是多少°．
<gadget id="calculator">180 - 80</gadget>
<output>100</output>

<gadget id="calculator">100 / 2</gadget>
<output>50</output>

<result>50</result>