The perimeter of an isosceles trapezoid is 17.9 cm, the upper base is 4.2 cm, the lower base is 5.7 cm, and the height is 3 cm. What is its waist in cm?
一个等腰梯形的周长是17.9厘米，上底是4.2厘米，下底是5.7厘米，高是3厘米，它的腰是多少厘米？
<gadget id="calculator">17.9 - 4.2 - 5.7</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<result>4</result>