The number of diagonals that can be drawn in a regular polygon with21:00前要,如果有算式要列算式!+_+是The number of diagonals that can be drawn in a regular polygon withtwenty sides(icosagon) is___________.

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The number of diagonals that can be drawn in a regular polygon with
21:00前要,如果有算式要列算式!+_+
是The number of diagonals that can be drawn in a regular polygon withtwenty sides(icosagon) is___________.

一个正多边形内能画的对角线的数目
前提n>=3,正多边形对角线数目(n*(n-3))/2
*表示乘
OK?

任意俩点之间有一连线
再减去边数就行了
n是顶点数：
n*(n-1)/2-n=n*n/2-3*n/2

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