A triangle has sides A, B, and C. Sides A and B have lengths of 3 and 2, respectively. The angle between A and C is (11pi)/24 and the angle between B and C is (pi)/8. What is the area of the triangle?

1 Answer
May 20, 2016

If A=3 then area is 7.57 and if B=2 area is 0.746

Explanation:

The third angle opposite sides A and B is

pi-(11pi)/24-pi/8=(24-11-3)pi/24=(10pi)/24=(5pi)/12 and it has side C opposite it.

As side A=3 has angle opposite it pi/8 and C has opposite to it angle (5pi)/12. Now, using sine formula, we get

3/sin(pi/8)=C/sin((5pi)/12) or

C=3xxsin((5pi)/12)/sin(pi/8)=3xx0.9659/0.3827=7.57

Hence area of triangle is 1/2xx3xx7.57xxsin((11pi)/24)

= 1/2xx3xx7.57xx0.9914=11.26

We have not used B=2 and as angle opposite it is ((11pi)/24), using sine formula

2/sin((11pi)/24)=C/sin((5pi)/12)

or C=2xxsin((5pi)/12)/sin((11pi)/24)=2xx0.9659/0.9914=1.95

and area of triangle is 1/2xx2xx1.95xxsin(pi/8)=1.95xx0.3827=0.746

Why this dichotomy? The fact is that we need either (a) one side and both angles on it; or (b) two sides and included angle and (iii) three sides of a triangle to identify a triangle and find area or other sides and angles of a triangle. However here we have been given four parameters and they give two different results depending on whether we take side A=3 or B=2 into consideration. In short, given three angles (third is derivable from other two), the two sides are not compatible and in fact refer to two different triangles.