Slopes of Parallel Lines 平行綫的斜率

http://tube.geogebra.org/material/show/id/177015 (Download GeoGebra File
Drag the red point to make the two lines parallel.
What is the relation of the slopes of parallel lines?

拖拉紅點,令兩綫平行。
平行綫的斜率有甚麼關係?

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