Exercises 18 and 19 are based on a theorem ( not stated) that is the converse of Theorem 5.6.3. See the figure on above.
Given: 

Find: 

(HINT:


To find:
Given:
Theorem used:
Angle bisector theorem:
If a ray bisects one angle of a triangle, then it divides the opposite side into segments whose lengths are proportional to the lengths of the two sides that form the bisected angle.
Calculation:
Given:
Then by anglebisector theorem, the ray
In a triangle the sum of the angles is
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