Question

i still dont understand this :,(
can someone help me to prove this identity?
I give medals!

Answer 1

what identity?

Answer 2

What's the question?

Answer 3

\[\cos (\frac{ \pi }{ 4 } +x) + \cos (\frac{ \pi }{ 4 } -x) = \sqrt{2} cosx\]

Answer 4

expand the left side using cosine addiction/difference formula

cos ( u + v) = cos u cos v - sin u sin v
cos ( u - v) = cos u cos v + sin u sin v

Answer 5

cos ( Pi/4 + x ) = cos (pi/4) cos x - sin(pi/4) sin x
cos ( Pi/4 - x ) = cos (pi/4) cos x + sin(pi/4) sin x

now add those two equations

Answer 6

okay thanks I'm looking at the problems and comparing what I have already

Answer 7

so what would I end up doing next? this is the part that I got stuck at

Answer 8

Okay, so sorry my brain is fried looking at this all day. It looks like you're saying to use a sum and difference formula for cosine

Answer 9

@jordanloveangel

Answer 10

@kittenlover731

Answer 11

@zepdrix

Answer 12

Cos(π/4+x)=cosπ/4.cosx-sinπ/4.sinx

Answer 13

huh?

Answer 14

Use my formula to prove ur identity

Answer 15

your formula didnt show up in the post, only question marks

Answer 16

refresh the page

Answer 17

okay

Answer 18

i think thats what @perl said above, she said to add those two equations

Answer 19

thats where i got stuck, i dont know what to do after that

Answer 20

@shamim what did you get after you added them?

Answer 21

@sophiaedge

Answer 22

2cos pi/4.cosx

Answer 23

im lost :( can you explain how you did that?

Answer 24

sorry grace, we most definitely haven't gone this far in my class. we're just wrapping up on the 6 functions

Answer 25

No worries, thanks for trying