Our teacher set us this problem: A triangle A B C has the following dimensions: I used tan with BC over AC and got the
answer of 51.34°. Why is Trigonometry not working? |
If you have a glance at a sketch of the triangle, and study the question, you will notice that the triangle is not necessarily right-angled.
It is important not to assume that the triangle has a right angle, unless you are specifically told it is.
The trigonometrical ratios sin, cos and tan,
and Pythagoras' Rule, only work for Right-Angled Triangles. |
In this case, you must use the Cosine Rule:
This re-arranges to
and substituting a=15, b=12, c=18 leads to the answer 55.8° (to 1 d.p.)