A triangle has sides A, B, and C. The angle between sides A and B is (5pi)/6 and the angle between sides B and C is pi/12. If side B has a length of 21, what is the area of the triangle?

1 Answer
Bdub
Mar 6, 2016

angleB =pi-((5pi)/6 +pi/12) = pi/12, a/sin(pi/12) = 21/sin(pi/12) -> a = 21/sin(pi/12) * sin(pi/12) = 21
Area = 1/2 ab sin C = 1/2 (21)(21)sin( (5pi)/6) = 110.25 unit^2

Explanation:

First, add the two given angles and subtract from pi to get the third angle. Then find one of the other two sides. In this case I found side a by using the sine rule. So to find the area of the triangle I use side a and b and angle C and substitute in to the angle formula then calculate.

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