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

    0
    ReputationRep:
    I've been doing some reviison for a test tomorrow and have come across this question. Show that the stationary point on  y=ax^2 +bx +c has coordinates (-b/2a, 4ac-b^2/4a) the cooriantes are -b all over 2a and 4ac- b^2 all over 4a

    I think its the algebraic brackets that are confusing me
    • PS Helper
    • Wiki Support Team
    Offline

    5
    ReputationRep:
    Differentiate it
    When dy/dx = 0 there is no gradient therefore it is a stationary point
    so put 0 = (what you get when you differentiate)
    and solve as you would a simultaneous equation

    You should get x = -b/2a and y=4ac-b^2/4a
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by greeneyedgirl)
    Differentiate it
    When dy/dx = 0 there is no gradient therefore it is a stationary point
    so put 0 = (what you get when you differentiate)
    and solve as you would a simultaneous equation

    You should get x = -b/2a and y=4ac-b^2/4a
    I did try this origannly i differentiated it to get 2ax +bx=0 but i dont see how if i take the second dirrivative it would lead to the asnwer
    • PS Helper
    • Wiki Support Team
    Offline

    5
    ReputationRep:
    (Original post by hazbaz)
    I did try this origannly i differentiated it to get 2ax +bx=0 but i dont see how if i take the second dirrivative it would lead to the asnwer
    You don't want to take the second derivative!
    d^2y/dx^2 is only used for finding whether the stationary point is a max or a min, not for determining the actual stationary point.
    Offline

    2
    ReputationRep:
    (Original post by hazbaz)
    I did try this origannly i differentiated it to get 2ax +bx=0 but i dont see how if i take the second dirrivative it would lead to the asnwer
    I think you should check that part again.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by greeneyedgirl)
    You don't want to take the second derivative!
    d^2y/dx^2 is only used for finding whether the stationary point is a max or a min, not for determining the actual stationary point.
    Opps silly me
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by dknt)
    I think you should check that part again.
    2ax +b ah right yh it makes sense now thanks
 
 
 
  • 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
    What newspaper do you read/prefer?
    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

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