Question

Use the Double-Angle Formulas to rewrite cos4x in terms of cos x
HELP. I knw the steps but ill explain more in comments...

Answer 1

I get to 2(cos2x)^2-1...

Answer 2

@primeralph :/ please

Answer 3

This is the kind of thing where you just keep going until you're done. Not hard.

Answer 4

but i dont see how you do the next step

Answer 5

the next thing i should get is...2(2cos2x - 1)^2 - 1

Answer 6

Recall that

\[\cos(A+B) = \cos(A)\cos(B) -\sin(A)\sin(B)\]

so cos(4x) = cos (2x + 2x) right? now substitute. and solve

Answer 7

Do the same for cos2x.

Answer 8

how is my question

Answer 9

Tell me what you get.

Answer 10

ok...thinking

Answer 11

2(cos^2u-1)-1

Answer 12

oh parenthhesis squared

Answer 13

u?

Answer 14

x

Answer 15

sorry....

Answer 16

Then expand or something.

Answer 17

so then 2(4cos^4x-4cos^2x+1)-1

Answer 18

distribute...

Answer 19

8cos^4x-8cos^2+1?

Answer 20

that was the answer :D

Answer 21

i wish i could figure this out myself...thanks guys

Answer 22

Hold on.

Answer 23

ok

Answer 24

Let me see all the options.

Answer 25

that was the only option..it wasnt for a test..i dont cheat like that...this was me trying to understand an example..

Answer 26

that was the answer provided

Answer 27

Okay.

Answer 28

thank you :)

Answer 29

You're welcome.