The coordinates of point D are (1, 10).
What does a midpoint mean?
Midpoint, as the word suggests, means the point which lies in the middle of something.
Midpoint of a line segment means a point which lies in the mid of the given line segment.
Given; CD has endpoints C and D, with C at coordinates (5,8). CD has midpoint M at (3,9).
Using midpoint formula;
Mₓ = (x₁ + x₂) / 2
3 = (5 + x) / 2
6 = 5 + x
x = 1
Now, Mᵧ = (y₁ + y₂) / 2
9 = (8 + y) / 2
18 = 8 + y
y = 10
Hence, The coordinates of point D are (1, 10).
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