1) The line segment we want to select must contain AB, so there is AB? 、AC？ Ad (article 3)

2) The line segment we want to select must contain BC, so there is BC? ，BD(2)

3) The line segment we want to select must contain CD, so there is CD( 1).

What I want to emphasize here is that line segment AC and line segment CA are one line segment.

Here is a simple way, that is, look at the picture. How many simple line segments are there? As shown above, it consists of three line segments AB. , BC? , CD, then the final result is 3+2+ 1=6.

Another example is10 (10 = 4+3+2+1).

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