Hey there Sign in to join this conversationNew here? Join for free

Maths past paper question

Announcements Posted on
Study Help needs new mods! 14-04-2014
Post on TSR and win a prize! Find out more... 10-04-2014
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    Can anyone please give me advice on how to work out the following question:

    Here is an identity:

    (2x + a)(x + 3) == 2X^2 + 4ax +b

    a and b are numbers.

    Work out b.
    I got the first line right by multiplying out the first part with the brackets as follows:

    2x^2 + 6x + ax + 3a

    Any idea on what to do next?

    Any help is appreciated as I have an exam tomorrow.
    • 0 followers
    Offline

    ReputationRep:
    Compare coefficients.
    From observation you can see that 6x + ax = 4ax which you can then solve to find A.
    You can then see that the constant term b is the product of 3 and A so 3a = b.
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    For the 6x + ax = 4ax part, you have to solve it by grouping all the 'a parts' on one side don't you, and then factorise it?

    So you'd do this:

    6x +ax = 4ax
    6x = 3ax
    0 = 3ax - 6x
    0 - 3x(a - 2)

    is that right?
    • 81 followers
    Online

    ReputationRep:
    (Original post by Sir_Malc)
    For the 6x + ax = 4ax part, you have to solve it by grouping all the 'a parts' on one side don't you, and then factorise it?

    So you'd do this:

    6x +ax = 4ax
    6x = 3ax
    0 = 3ax - 6x
    0 - 3x(a - 2)

    is that right?
    Yes, so what do you know about the expression a-2?
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    a = 2?
    • 81 followers
    Online

    ReputationRep:
    (Original post by Sir_Malc)
    a = 2?
    Yep :yep:
    • Thread Starter
    • 0 followers
    Offline

    ReputationRep:
    ah, so you substitute that back into the original quadratic and you end up with b = 6 at the end.

    Thank you very much both of you for the help.

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?

    this is what you'll be called on TSR

  2. this can't be left blank
    this email is already registered. Forgotten your password?

    never shared and never spammed

  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. By completing the slider below you agree to The Student Room's terms & conditions and site rules

  2. Slide the button to the right to create your account

    Slide to join now Processing…

    You don't slide that way? No problem.

Updated: March 6, 2012
Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.