NCERT Solutions
Class 11 Maths
Introduction to Three Dimensional Geometry

Ex.Misc. Q.2
Find the lengths of the medians of the triangle with vertices A (0, 0, 6), B (0, 4, 0) and (6, 0, 0).
Let AD, BE, and CF be the medians of the given triangle ABC.
Since AD is the median, D is the mid-point of BC.
So, coordinates of point D = {(0 + 6) ÷ 2, (4 + 0) ÷ 2, (0 + 0) ÷ 2} = (3, 2, 0)
AD = √ {(0 - 3)2 + (0 - 2)2 + (6 - 0)2} = √ (9 + 4 + 36) = √49 = 7
Since BE is the median, E is the mid-point of AC.
So, coordinates of point E = {(0 + 6) ÷ 2, (4 + 0) ÷ 2, (6 + 0) ÷ 2} = (3, 0, 3)
BE = √ {(3 - 0)2 + (0 - 4)2 + (3 - 0)2} = √ (9 + 16 + 9) = √34
Since CF is the median, EF is the mid-point of AB.
So, coordinates of point F = {(0 + 0) ÷ 2, (0 + 4) ÷ 2, (6 + 0) ÷ 2} = (0, 2, 3)
Length of CF = √ {(6 - 0)2 + (0 - 2)2 + (0 - 3)2} = √ (36 + 4 + 9) = √49 = 7
Thus, the lengths of the medians of ∆ABC are 7, √34 and 7.