In an isosceles triangle, the ratio of the vertex angle to the base angle is 1:4, what is the base angle of this triangle?
一个等腰三角形，顶角与一个底角度数的比是1：4，这个三角形的底角是多少度．
<gadget id="calculator">1 + 4 + 4</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">180 * (4/9)</gadget>
<output>80</output>

<result>80</result>