AB/AC = BD/CD = [ABD]/[ADC] = [1/2 (AB AD sin(x))]/[1/2 (AD AC sin(y))]
AB/AC = [1/2 (AB AD sin(x))]/[1/2 (AD AC sin(y))]
Simplifying, we get:
sin(x) = sin(y) = sin(180-x)
Angles of a triangle are in measure between 0 and 180.
sin(x) can be equal to sin(y) in the range 0 - 180 only when either x = y or x = 180 - y [I have concluded this].
x = y is a not abnormal case, so let's look at the x = 180 - y case.
If x = 180 - y, then x + y = 180, which will make angle BAC 180 deg. This implies the other two angles are 0. But side/sin(angle opposite to the side) = 2R
side = 2R * sin(0) = 0
If x + y = 180 deg were to be true, then AC and AB would both have to be 0 in length. The initial ratio 0/0 would be therefore undefined, which is a contradiction to our assumption that the laws of physics are valid. So only the x = y case is correct.
So AD is the angle bisector of angle BAC.
Is this proof correct (of the converse of the angle bisector theorem)?