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I'm assuming that you're after the dragging feature of a slider (or clicking on and snapping to a part within it).
The slider bar is a segment between two points A and B. Your touch is "above" or "below" said segment when you care about it. You can draw another segment from this point to the bar, which will be at a 90-degree angle to the bar itself.
Finally, you can draw another segment between either A or B and your touch point.
This is a right triangle. Say we chose A. From A to the point along the bar you have one side, going "up" from there to your touch is another. From A to the touch you have the hypotenuse.
At this point you're basically dealing with projection and / or rejection. (A little tired at the moment but could elaborate more later, or maybe somebody else will chime in.)
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