2y = 2x + b (exterior angle = the sum of the other two interior angles)
--- equation I
y = x + a (same reasoning as above)
--- equation II
Plug in the first equation and you have
2y = 2x + 2a = 2x + b
2a = b
Here is the link to the Angle Bisector Theorem, including the proof and one example.
Angle ABC and ACB are both trisected into three congruent angles of x and y respectively.
If given angle "a" value, find angle c and angle b.
Solution: 3x + 3y = 180 - a
From there, it's very easy to find the value of x + y
and get angle c, using 180 - (x + y).
Also, once you get 2x + 2y, you can use the same method -- 180 - (2x + 2y) to get angle b
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