Find the missing endpoint of a line segment from one known endpoint and the midpoint.
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Result
Missing endpoint
Segment length
5.6569
Half length
2.8284
Also in General
Geometry
The endpoint calculator finds the missing endpoint of a line segment when you know one endpoint and the midpoint. It reverses the midpoint formula to solve for the unknown point.
If M is the midpoint and A is the known endpoint, then the unknown endpoint B = 2M - A. This works for each coordinate independently: Bx = 2*Mx - Ax, By = 2*My - Ay.
The formula is derived by rearranging the midpoint equation M = (A+B)/2 to solve for B.
B = 2M - A
The missing endpoint equals twice the midpoint minus the known endpoint.
This calculator assumes the midpoint divides the segment in a 1:1 ratio. For other division ratios, use the section formula.
Frequently asked questions
(7, 8). Bx = 2(4) - 1 = 7. By = 2(5) - 2 = 8.
Yes — compute the midpoint of the known endpoint and your answer. It should equal the given midpoint.
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