If we label the points ABCD as given, then
AB and CD have slope -3/2
AD and BC have slope 1/2
so, the sides are parallel
AC is vertical
BD has slope -1/2
so, tanθ = 2 ==> θ = 63.4°
The calculations are easier if you plot the points and draw the diagonals.
prove that the points (2,-1) (0,2) (2,3)and (4,0) are the coordinates of vertices of a parallelogram and find the angle between its diagonals
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