The perimeter of an isosceles triangle is b centimeters, and the base is 12 centimeters long. Use the formula to express the waist length. When b=40, find the waist length in centimeters.
等腰三角形的周长是b厘米，底边长12厘米，用式子表示腰长．当b=40时，求腰长是多少厘米．
<gadget id="calculator">40 - 12</gadget>
<output>28</output>

<gadget id="calculator">28 / 2</gadget>
<output>14</output>

<result>14</result>