Given that the base angle of an isosceles triangle is 40 degrees, what is the apex angle of this isosceles triangle?
已知一个等腰三角形的底角是40度，这个等腰三角形的顶角是多少度？
<gadget id="calculator">180 - 40 - 40</gadget>
<output>100</output>

<result>100</result>