Just one sec...
Hey! Sign in to get help with your study questionsNew here? Join for free to post

Integration (C4) 4sin2xcos2x

Announcements Posted on
GCSE grade boundaries are here - GO> 23-08-2016
    • Thread Starter
    Offline

    ReputationRep:
    Integration of 4sin2x.cos2x dx

    Not sure how to do this but I tried by substitution and got -cos^2(2x)+c

    u=cos2x
    du/dx=-2sin2x
    dx/du=-1/2sin2x
    sin2x(dx/du)=-1/2
    But arrived at the wrong answer is there a quicker qay of doing this?
    Offline

    ReputationRep:
    I would let u= sin2x

    du/dx = 2cos2x

    du = dx*2cos2x.
    Offline

    ReputationRep:
    Another way of doing this question is to notice that sin(4x)=sin(2x+2x)=2sin(2x)cos(2 x) then using that to make a trig substitution.

    However, the answer you got is actually correct. If you try my method you get (-1/2)cos(4x) + k which can be manipulated to show that (-1/2)cos(4x) differs from -cos^2(2x) by a constant and hence they are equivilant answers to the question.
    Offline

    ReputationRep:
    (Original post by Luppy021)
    Integration of 4sin2x.cos2x dx

    Not sure how to do this but I tried by substitution and got -cos^2(2x)+c

    u=cos2x
    du/dx=-2sin2x
    dx/du=-1/2sin2x
    sin2x(dx/du)=-1/2
    But arrived at the wrong answer is there a quicker qay of doing this?




    Uploaded with ImageShack.us

    There are several ways of doing this problem and they might get answers that LOOK different.


    EDIT

    Why the rep?
    • Thread Starter
    Offline

    ReputationRep:
    Yeah I get it now cheers all
    Offline

    ReputationRep:
    Your answer is correct. I think it will differ by a constant from the book answer.
    Offline

    You could also use integration by parts

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: April 5, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Poll
How do you sleep?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read here first

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
Study resources

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.