SAT II Math II : Finding Angles with Trigonometry

Study concepts, example questions & explanations for SAT II Math II

varsity tutors app store varsity tutors android store

Example Questions

Example Question #2 : Trigonometry

In \(\displaystyle \bigtriangleup ABC\):

\(\displaystyle AB = 23\)

\(\displaystyle AC = 34\)

\(\displaystyle BC = 25\)

Evaluate \(\displaystyle m \angle A\) to the nearest degree.

Possible Answers:

\(\displaystyle 43^{\circ }\)

\(\displaystyle 27^{\circ }\)

\(\displaystyle 90^{\circ }\)

\(\displaystyle 63^{\circ }\)

\(\displaystyle 47^{\circ }\)

Correct answer:

\(\displaystyle 47^{\circ }\)

Explanation:

The figure referenced is below: 

Triangle z

 

 

By the Law of Cosines, the relationship of the measure of an angle \(\displaystyle \gamma\) of a triangle and the three side lengths \(\displaystyle a\)\(\displaystyle b\), and \(\displaystyle c\)\(\displaystyle c\) the sidelength opposite the aforementioned angle, is as follows:

\(\displaystyle c^{2} = a ^{2} + b^{2} - 2ab \cos \gamma\)

All three side lengths are known, so we are solving for \(\displaystyle \gamma\). Setting

\(\displaystyle c = BC = 25\), the length of the side opposite the unknown angle;

\(\displaystyle a = AB = 23\);

\(\displaystyle b = AC = 34\);

and \(\displaystyle \gamma = \cos A\),

We get the equation

\(\displaystyle 25^{2} =23^{2} +34^{2} - 2 (23) (34)\cos A\)

\(\displaystyle 625 =529+1,156- 1,564 \cos A\)

\(\displaystyle 625 =1,685 - 1,564 \cos A\)

Solving for \(\displaystyle \cos A\):

\(\displaystyle 625 - 1,685 =1,685 - 1,564 \cos A - 1,685\)

\(\displaystyle -1.060 = - 1,564 \cos A\)

\(\displaystyle \frac{-1.060 }{- 1,564}= \frac{- 1,564 \cos A}{- 1,564}\)

\(\displaystyle \cos A \approx 0.6777\)

Taking the inverse cosine:

\(\displaystyle A = \cos ^{-1} 0.6777 \approx 47 ^{\circ }\),

the correct response.

Learning Tools by Varsity Tutors