Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    2
    ReputationRep:
    This question is from C2 Edexcel June 2013, Q8 part (ii) (b)

    I let cos theta = x for typing faster.

    4x^2 + 2x -1 = 0

    I used completing the square

    4(x^2+x/2)-1=0

    4(x+1/4)^2-(1/16)-1=0

    4(x+1/4)^2=17/16

    (x+1/4)^2=17/64


    (x+1/4)= root (17/64)

    x=-1/4 plus or minus root (17/64)


    That gives x, or cos theta, = 0.265.... or =-0.265....

    This isn't correct, can anyone please tell me what I've done wrong?
    Offline

    22
    ReputationRep:
    (Original post by jamb97)
    This question is from C2 Edexcel June 2013, Q8 part (ii) (b)

    I let cos theta = x for typing faster.

    4x^2 + 2x -1 = 0

    I used completing the square

    4(x^2+x/2)-1=0

    4(x+1/4)^2-(1/16)-1=0

    4(x+1/4)^2=17/16
    Bolded bits. You should review completing the square again or how to expand brackets.

    4\bigg((x + 0.25)^2 - \frac{1}{16}\bigg) - 1 = 0 \iff 4(x+0.25)^2 - \frac{4}{16} - 1 = 0

    This gives  4(x + 0.25)^2 = \frac{5}{4} \neq \frac{17}{16}
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by Zacken)
    Bolded bits. You should review completing the square again or how to expand brackets.

    4\bigg((x + 0.25)^2 - \frac{1}{16}\bigg) - 1 = 0 \iff 4(x+0.25)^2 - \frac{4}{16} - 1 = 0

    This gives  4(x + 0.25)^2 = \frac{5}{4} \neq \frac{17}{16}
    Woops... Got it. Thanks very much
    Offline

    22
    ReputationRep:
    (Original post by jamb97)
    Woops... Got it. Thanks very much
    Using big brackets when you have nested expressions like these are very useful for avoiding these types of mistakes.
    Offline

    4
    ReputationRep:
    (Original post by jamb97)
    -
    Alternatively you could just use the quadratic formula to avoid silly mistakes.
    Offline

    22
    ReputationRep:
    (Original post by edothero)
    Alternatively you could just use the quadratic formula to avoid silly mistakes.
    Or make more silly mistakes using the quadratic formula.
    Offline

    4
    ReputationRep:
    (Original post by Zacken)
    Or make more silly mistakes using the quadratic formula.
    I can relate to that statement :laugh:
    Offline

    22
    ReputationRep:
    (Original post by edothero)
    I can relate to that statement :laugh:
    Me 2 years ago in GCSE:

    2 kul for u bby imma use quadratic formula

    #negative discriminant

    imaginary numbers betch
    Offline

    4
    ReputationRep:
    (Original post by Zacken)
    Me 2 years ago in GCSE:

    2 kul for u bby imma use quadratic formula

    #negative discriminant

    imaginary numbers betch
    lmaooo I remember once when I was ill I had a c3 mock, started using complex numbers thinking I was doing fp2. Facepalms were real
    Offline

    22
    ReputationRep:
    (Original post by edothero)
    lmaooo I remember once when I was ill I had a c3 mock, started using complex numbers thinking I was doing fp2. Facepalms were real
    :rofl:
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Would you rather give up salt or pepper?
    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
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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.