Join TSR now and get all your revision questions answeredSign up now

Help with this question please!!

    • Thread Starter
    Offline

    1
    ReputationRep:
    Prove that 1+ cos2x + cos4x = cos2x (4cos^2x - 1)

    I've tried this question so many times and I can't seem to do it! Help pls
    Offline

    3
    ReputationRep:
    (Original post by rn123)
    Prove that 1+ cos2x + cos4x = cos2x (4cos^2x - 1)

    I've tried this question so many times and I can't seem to do it! Help pls
    By inspection you can see that if you leave the \cos(2x) alone for now and only deal with 1+\cos(4x) you will be able to factor it out later on once you turn the 4x into 2x.

    So apply the double formulae for cosine: \cos(2A)\equiv 2\cos^2(A)-1 and simplify from there.

    Hint if you get stuck:
    Spoiler:
    Show
    You will need to apply this twice.
 
 
 
Poll
Do you have exam superstitions?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.