The angle bisector theorems is proved below.
It should be noted that the angle bisector theorem simply states that an angle bisector of a triangle divides the opposite side into two segments which are proportional to the other sides of the triangle.
The way to proof the theorem is illustrated:
Draw a ray CX parallel to AD and then extend BA to intersect this ray at E.
In triangle CBE, DA is parallel to CE.
BD/DC == BA/AE ......... i
Now we want to prove that AE = AC
Since DA is parallel to CE, we have:
DAB = CEA (corresponding angles) ....... ii
DAC = ACE (alternate interior angles) ...... iii
Since AD is the bisector of BAC, we've DAB = DAC.
From the above, ACE makes and isosceles triangle and since the opposite sides are equal, we've AC = CE.
Substitute AC for AE in equation i
BD/DC = BA/AC
Therefore, the angle bisector theorems is proved.
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