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

a level maths C1 Watch

Announcements
    • Thread Starter
    Offline

    9
    ReputationRep:
    Help please someone tell me how to do inequalities and also tell me how you know the sign is < or > . For example, x^2 -11x+24<0, why is the answer 3<x<8 and how do you do it please !!!!!! anwer
    • Very Important Poster
    Offline

    21
    ReputationRep:
    (Original post by Mr revision)
    Help please someone tell me how to do inequalities and also tell me how you know the sign is < or > . For example, x^2 -11x+24<0, why is the answer 3<x<8 and how do you do it please !!!!!! anwer
    You sketch the graph and look for the points where y < 0 or y> 0 etc.



    For what values of x is the y value less than 0?
    • Community Assistant
    Online

    18
    ReputationRep:
    (Original post by Mr revision)
    Help please someone tell me how to do inequalities and also tell me how you know the sign is < or > . For example, x^2 -11x+24<0, why is the answer 3<x<8 and how do you do it please !!!!!! anwer
    You have to find the critical values.

    x^2 - 11x + 24 factorises to (x-3)(x-8)

    draw that curve and look for the region where y is less than 0.
    Offline

    14
    ReputationRep:
    (Original post by SeanFM)
    You sketch the graph and look for the points where y < 0 or y> 0 etc.



    For what values of x is the y value less than 0?
    Spoiler:
    Show
    i was going to answer that
    Spoiler:
    Show
    you always hack on maths threads :emo:
    Spoiler:
    Show
    jokes :lol:
    Spoiler:
    Show
    no i wasn't :fuhrer:
    Offline

    3
    ReputationRep:
    (Original post by Mr revision)
    Help please someone tell me how to do inequalities and also tell me how you know the sign is < or > . For example, x^2 -11x+24<0, why is the answer 3<x<8 and how do you do it please !!!!!! anwer
    first, set  \displaystyle x^2 - 11x + 24 = 0 and solve for x to find critical values.

    when you do this youll get x = 3 or x = 8.

    From here you need to work out whether the curve is less than 0 for 3<x<8 or x<3 U x>8. To do this you have two choices;

    Sketch it - you can see that the quadratic has a y intercept of 24 so you know three points on the curve are (0,24), (3,0) and (8,0). You also know this quadratic has a U shape so a quick sketch will tell you that  \displaystyle x^2 - 11x + 24 &lt; 0 for values 3 < x < 8.

    Or you can sub in some values - sub in x = 0, x = 5 and x = 10 for example and youll see that f(0) >0, f(5) < 0 and f(10) > 0 therefore the function is <0 for values 3<x<8
    • Thread Starter
    Offline

    9
    ReputationRep:
    Thank you all but how do you look for the region where y>0 or y<0
    • Thread Starter
    Offline

    9
    ReputationRep:
    (Original post by NotNotBatman)
    You have to find the critical values.

    x^2 - 11x + 24 factorises to (x-3)(x-8)

    draw that curve and look for the region where y is less than 0.
    Thanks but how do you look for the region where y is less than 0
    • Community Assistant
    Online

    18
    ReputationRep:
    (Original post by Mr revision)
    Thanks but how do you look for the region where y is less than 0
    All the points of the curve below the x axis. If you look at the graph SeanFM posted, y=0 on the x axis, so at x=3 and x=8, anything below that is where y<0 and anything above is where y>0.
 
 
 
  • 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
    Break up or unrequited love?
    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.