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
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.)