Determine the measure of angle B of triangle ACB, given the following information: A=69degrees, a=25feet, b=16feet
This is called the "ambiguous case", when you
are given two sides and an angle which is not
included between them. The ambiguous case has
0, 1, or 2 solutions.
We use the law of sines:
a b c
-------- = -------- = --------
sin(A) sin(B) sin(C)
Here we only need:
a b
-------- = --------
sin(A) sin(B)
25 16
--------- = --------
sin(69°) sin(B)
Cross multiply:
25·sin(B) = 16·sin(69°)
Divide both sides by 25 to solve for sin(B)
16·sin(69°)
sin(B) = -------------
25
sin(B) = .597491473
1. Sometimes this step comes our greater than 1.
If so, there are no solutions.
2. Sometimes this step comes out exactly 1. If so
then B is a right angle and there is 1 solution.
3. Usually this step comes out a positive number
between 0 and 1. If this is the case, then there
are either one or two solutions. We must check to
see if there are two solutions.
(a) Find the first quadrant solution for angle B
sin(B) = .597491473
Find the inverse sine of that right side:
B = 36.6094481°
Since the sine is positive in the 2nd quadrant, there
is another solution for B, namely its supplement:
B = 180 - 36.6094481° = 143.3095519°
Now we calculate what angle C would have to be if that
were a possible solution for angle B.
A + B + C = 180°
69° + 143.3095519° + C = 180°
212.3095519° + C = 180°
C = 180° - 212.3095519
C = -32.3095519°
C cannot be a negative angle, so there is but
one solution, namely
B = 36.6094481°
Since the sides are only given to two significant
digits, we should round the answer to the nearest
degree:
B = 37°
Edwin