00:04
Okay, here's a fun problem.
00:06
You're going to have to remember a couple of your trig properties here.
00:11
Well, one in particular, of course.
00:14
But to find the exact value of a trigometric expression means, you know, you shouldn't need a calculator.
00:22
You're going to use your common ratios and whatever.
00:27
But you'll notice that 80 degrees isn't a common angle, nor is 70 degrees.
00:32
So that tells us there's got to be something we got to do to make this that we can find the exact value without a calculator.
00:40
Well, looking at this expression, it kind of has this form, right? cosign of some angle a times cosine of some other angle b minus sign of the first angle a times sign of the second angle b.
00:59
Sorry, let me use a and b first.
01:03
So, sine a, sign b.
01:06
And if you have some sort of trig reference sheet, or if you have any notes out, you might recognize that this is the expansion of the cosine of a sum of two angles...