Question 212913
Since, b is the midpoint of ac then a, b, and c are collinear (the points lie on the same line)
Since, c is the midpoint of bd then b, c, and d are collinear as well.
So, a, b, c, and d are collinear.
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Since b is the midpoint of ac, then ab=bc
Since c is the midpoint of bd, bc=cd.
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Now, since ab=bc and bc=cd, then
ab=cd