Question

In triangle ABC, a=4, b=7, c=9. Find the measure of angle C to the nearest tenth of a degree.

My progress:

9^2=7^2+4^2-2(9)(7)cosC
81=49+16-126cosC

81=65 - 126cosC ?

I am unsure how to evaluate it.

Answer 1

\[
\begin{align*}
81&=65-126\cos C \\
16&=-126\cos C \\
\frac{-8}{63}&=\cos C
\end{align*}
\]

Answer 2

-0.1269?

Answer 3

Oh 2nd cos button?

Answer 4

Use the inverse cosine function.

Answer 5

What is your answer?