数学题：如图半圆的直径为20厘米，已知阴影A比阴影B的面积少27平方厘米。求MN的长是多少?

数学题解答：

A比B的面积少27平方厘米，A和B都加上直径上方的空白部分，就分别变成了三角形和半圆。

所以三角形比半圆少27平方厘米，根据这一点可以列方程解决。

设MN为x厘米。

等量关系式：三角形面积 + 27 ＝ 半圆面积

20x÷2 + 27 = 3.14×(20÷2)²÷2

10x+27=157

x=13

答：MN的长是13厘米。

算术解法：

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