Turn on thread page Beta
    • Thread Starter
    Offline

    16
    ReputationRep:
    Name:  Screenshot_2016-03-27-17-13-43.png
Views: 161
Size:  141.1 KB for part d of this question. I got to the part 'k^2-12<0 ' and I don't understand why you can just add 12 on both side and just square root 12. I was told that you can't square root numbers when there is an inequality sign there. Thanks.
    Offline

    3
    ReputationRep:
    (Original post by coconut64)
    Name:  Screenshot_2016-03-27-17-13-43.png
Views: 161
Size:  141.1 KB for part d of this question. I got to the part 'k^2-12<0 ' and I don't understand why you can just add 12 on both side and just square root 12. I was told that you can't square root numbers when there is an inequality sign there. Thanks.
    you can and you get 2 answers
    Offline

    15
    ReputationRep:
    Sketch a graph of y=k2-12 and you will see that for y<0 , k has to be between 2 values.  -2\sqrt 3 &lt; k &lt; 2\sqrt 3 .
    • Thread Starter
    Offline

    16
    ReputationRep:
    (Original post by B_9710)
    Sketch a graph of y=k2-12 and you will see that for y<0 , k has to be between 2 values.  -2\sqrt 3 &lt; k &lt; 2\sqrt 3 .
    Thats true. Thats in one of the past paper. I vaguely remembered my teacher telling me that you can't square root it when it has an inequality sign though. So when can you square it root and when can you not? thanks.
    Offline

    15
    ReputationRep:
    (Original post by coconut64)
    Thats true. Thats in one of the past paper. I vaguely remembered my teacher telling me that you can't square root it when it has an inequality sign though. So when can you square it root and when can you not? thanks.
    Well have a little think about it. If we have  x^2 &lt; 1 you cannot just square root it to get  x&lt;1 . You have to think about the graph of  y=x^2 and realise that this means that  y&lt;1 and from the graph you can clearly see that this only occurs when  -1&lt;x&lt;1 .
    Generally when inequalities are involved you have to take extra caution than in an equation.
    You would have to take special care when solving
     \displaystyle \frac{x}{x-5} &lt; x+2 (random example). In this example you could not just multiply by (x-5) as if (x-5) was negative then it would flip the inequality sign. You do not need to worry about the second example I gave this will not be tested in standard A level maths.
    Main point is take extra care when dealing with inequalities.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: March 28, 2016

University open days

  1. University of Cambridge
    Christ's College Undergraduate
    Wed, 26 Sep '18
  2. Norwich University of the Arts
    Undergraduate Open Days Undergraduate
    Fri, 28 Sep '18
  3. Edge Hill University
    Faculty of Health and Social Care Undergraduate
    Sat, 29 Sep '18
Poll
Which accompaniment is best?
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

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.