5:07 Distance between two coordinates - Corbettmaths
Corbettmaths shows the exact GCSE method for the distance between two coordinates.
Find the length of a line segment in the coordinate plane
Calculate the length of a line segment on the coordinate plane
4 hand-picked video lessons for this topic, each checked for relevance and clarity, graded by difficulty and labelled by depth. Press play to watch — nothing loads from YouTube until you do.
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4 shown 5:07 
2:54 Finding the length of a line segment
Clear, concise walkthrough of finding a line segment's length from its endpoint coordinates.
9:13 The Maths Prof: Length of Line Segment
Full method plus a reverse problem where you're given the length instead.
5:21 Using Distance Formula to Find Distance Between Two Points!
Uses the distance formula, clearly linking it back to Pythagoras' theorem.