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<img alt="" src="example1.png">
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<ul class="menu"><li>set <kbd>F4</kbd> and <kbd>F5</kbd> snapping on, go into <em>line</em> mode, and
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<li>go near <i>p</i>, press <kbd>F1</kbd> and <kbd>F8</kbd> to set origin and
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<li>go near <i>p'</i>, click left, and extend segment to <i>s<sub>2</sub></i>.