Turn on thread page Beta
    • Thread Starter
    Offline

    2
    ReputationRep:
    My brain is dead after not thinking for 4 weeks and part of the answer in the back of the book doesn't make sense to me.

    Q. Prove the identity

    cos(A+B)cos(A-B)=cos2A-sin2B

    The solution given is this. The bit I don't understand is in bold.

    LHS=[cosAcosB-sinAsinB]*[cosAcosB+sinAsinB]

    =cos2Acos2B-sin2Asin2B

    =cos2Acos2B-(1-cos2A)sin2B

    =cos2A(cos2B+sin2B)-sin2B

    =cos2A-sin2B

    =RHS

    I can't work out where that comes from the previous step.

    Excuse me being thick.
    Offline

    9
    ReputationRep:
    They have just expanded it and then factorised.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by CalculusMan)
    They have just expanded it and then factorised.
    Oh I get it now. Thanks.

    I told you my brain was dead.
    Offline

    10
    ReputationRep:
    Multiply out the bracket

    cos2Acos2B-(1-cos2A)sin2B
    = cos2Acos2B - sin2B + sin2Bcos2A

    Take out a common factor of cos2A
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: August 20, 2006

University open days

  • University of Lincoln
    Mini Open Day at the Brayford Campus Undergraduate
    Wed, 19 Dec '18
  • University of East Anglia
    UEA Mini Open Day Undergraduate
    Fri, 4 Jan '19
  • Bournemouth University
    Undergraduate Mini Open Day Undergraduate
    Wed, 9 Jan '19
Poll
Were you ever put in isolation at school?
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

Equations

Best calculators for A level Maths

Tips on which model to get

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

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