Question

Find the value of the following using the concepts of both addition and subtraction formulas and reference angle:

cos 195

Answer 1

cos (195) = cos (180 + 15) = - cos (15)

Do you know how to find cos (15)?

Answer 2

cos (45-30) = cos 15?
@MonoPolyRoly

Answer 3

Yes.

Answer 4

what to do next @MonoPolyRoly

Answer 5

Use the compound angle formula cos(A-B) = cosAcosB + sinAsinB.

Answer 6

= ((sqrt6)/4) + ((sqrt2)/4)

?
@MonoPolyRoly

Answer 7

That's cos(15). Note that cos(195) = -cos(15). So...

Answer 8

what happens?

why the negative? @MonoPolyRoly

Answer 9

I did explain to you why cos(195) = -cos(15) in my first comment here. Please scroll up and check it. :)

Answer 10

i mean where did you get the -
from what derivation?
@MonoPolyRoly

Answer 11

= - ((sqrt6)/4) + ((sqrt2)/4)
is that it? @MonoPolyRoly

Answer 12

It is.
Also, cos (195) = cos (180 + 15) = - cos (15). Not that cos (180 + x) = -cos(x)