Question

cos(285[degrees])
Exact value?

Answer 1

just put it in the calculator? .2588190451

Answer 2

precal they need an exact value.

cos285 = cos75

Now you can use angle sum formulas, since cos75 = cos(45+30)

cos(A+B) = cosAcosB + sinAsinB

cos(30+45) = cos30cos45 + sin30sin45

And you can use an exact values chart for those and solve it.

This should help if you get stuck: http://www.mathwarehouse.com/trigonometry/identities/angle-sum-formula.php

Answer 3

Even/Odd Identities
cos(-75)=cos75