cos(80º)

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}{\left({\mathtt{80}}^\circ\right)} = {\mathtt{0.173\: \!648\: \!177\: \!667}}$$

Look up a trig. identity for cos (a + b) by searching for "trigonometric identities.'

Then use a = 90 degrees and b = -10 degrees.

cos 90 degrees = zero, so the expression is just equal to the cos(-10 degrees).

How you should answer this problem depends on what references the you are allowed to use.

"Old school" trigonometry books had tables for all the trig function values of each individual degree.

I have such a book. I could give you the answer to four decimal places and the reference for the book.

Because of this, you might search for a "Trigonometry Table."

Provide me with the references you are allowed to use, and I will search for links to help you find a solution.

John

Phill, do you know of better advice for this ?

http://www.sosmath.com/tables/trigtable/trigtable.html

This link provides the result, however no skill in math is involved.

This would still be a good reference to check your answer.

John

We might be able to use some kind of sum or difference identity, but it would be tough trying to find two such angles with known cosine and sine values.....we might know one, but the second would be as unknown as cos(80). For example, we know cos(90) and sin(90), but what's cos(10) and sin(10)??

My advice.....just put it into the calculator and evaluate....make sure you're in degree mode.

The online trig table that I cited  is in agreement with Melody.

Thanks, Melody