Question

How would i find cos(225) using sum and difference identities?

Answer 1

since \(225=180+45\) you could use that one

Answer 2

\[\cos(225)=\cos(180+45)=\cos(180)\cos(45)-\sin(180)\sin(45)\] is a start

Answer 3

180 is an obtuse angle, dont I have to use acute angles?

Answer 4

no it makes no difference at all

Answer 5

doubt you are going to find two acute angles that add up to \(225\) in any case

Answer 6

ok thank you (:

Answer 7

yw

Answer 8

so would my answer be - square root of 2 over 2