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

C3 Trigonometry using double angle formula Watch

    • Thread Starter
    Offline

    2
    ReputationRep:
    1) tan2A + 5tanA = 0
    find all values for A between 0 and 2pi
    I can find 0, pi and 2pi for answers but not the rest

    2) Prove the identity tan2x - tanx = tanxsec2x
    I can get as far as (2sinxcos^2x-sinxcos2x)/(cos2xcosx) but don't know where to go from there

    3)Prove that (cosA + sin A)(cos2A + sin2A) = cosA + sin3A
    After expanding brackets I'm not sure what to do next
    • Very Important Poster
    Online

    3
    ReputationRep:
    (Original post by marmbite)
    1) tan2A + 5tanA = 0
    find all values for A between 0 and 2pi
    I can find 0, pi and 2pi for answers but not the rest

    2) Prove the identity tan2x - tanx = tanxsec2x
    I can get as far as (2sinxcos^2x-sinxcos2x)/(cos2xcosx) but don't know where to go from there

    3)Prove that (cosA + sin A)(cos2A + sin2A) = cosA + sin3A
    After expanding brackets I'm not sure what to do next
    For 1, use double angle formulae and see if you can factor out something to solve it. (Never cancel things out by division!)

    For 3, think about how to apply addition formulae.
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by SeanFM)
    For 1, use double angle formulae and see if you can factor out something to solve it. (Never cancel things out by division!)
    did that, got half the answers but for some reason my other half were wrong and I can;t work out why
    I got to 10tan^A + 2tanA=0
    which became 2tanA(tanA + 1) = 0
    the answers I meant to be 0, 0.87, 2.27, pi, 4.01, 5.41, 2pi
    I can't work out how to get the 0.87 and therefore can't get half the answers

    (Original post by SeanFM)
    For 3, think about how to apply addition formulae.
    Tried this and got in a mess and couldn't see what to do with it, I'm not sure exactly where to apply the addition formulae, or should I just apply it to all the cos2As and sin2As?
    • Very Important Poster
    Online

    3
    ReputationRep:
    (Original post by marmbite)
    did that, got half the answers but for some reason my other half were wrong and I can;t work out why
    I got to 10tan^A + 2tanA=0
    which became 2tanA(tanA + 1) = 0
    the answers I meant to be 0, 0.87, 2.27, pi, 4.01, 5.41, 2pi
    I can't work out how to get the 0.87 and therefore can't get half the answers
    How did you get from tan2a to 10tan^2(A)?


    Tried this and got in a mess and couldn't see what to do with it, I'm not sure exactly where to apply the addition formulae, or should I just apply it to all the cos2As and sin2As?
    It's slightly 'cheating' (well, not really..) but it's helpful to look at the right hand side. Think of, in sin(a+b) and cos(a+b) etc, what the a and b can be to leave you ending up with cosA + sin3A
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by SeanFM)
    How did you get from tan2a to 10tan^2(A)?
    that's where I went wrong.
    Found the problem now and got the right answer, I'd made a silly error when dealing with the fraction

    (Original post by SeanFM)
    It's slightly 'cheating' (well, not really..) but it's helpful to look at the right hand side. Think of, in sin(a+b) and cos(a+b) etc, what the a and b can be to leave you ending up with cosA + sin3A
    I'll try this, thanks
 
 
 
Poll
Is GoT overrated?
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.