Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    • Thread Starter
    Offline

    12
    ReputationRep:
    I already know how to rationalise the denominator, I just want to know when

    For example, in this problem:
    Name:  grade 9 question maths.png
Views: 88
Size:  4.2 KB
    I have to add the numbers first. But in this problem:
    Name:  rationalise first.png
Views: 76
Size:  765 Bytes
    I have to rationalize the first fraction first. Can someone help me out and explain why? Thanks.
    Offline

    18
    ReputationRep:
    Is that when you times it all by 1 - (square root) 2 and multiple the whole thing out?
    Then after that times the whole thing by (square root 2) and you should get that answer
    Offline

    12
    ReputationRep:
    Can you see what would happen if you multiplied top and bottom by 1 - \frac{1}{\sqrt{2}}?
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by Gabzinc)
    I already know how to rationalise the denominator, I just want to know when

    For example, in this problem:
    Name:  grade 9 question maths.png
Views: 88
Size:  4.2 KB
    I have to add the numbers first. But in this problem:
    Name:  rationalise first.png
Views: 76
Size:  765 Bytes
    I have to rationalize the first fraction first. Can someone help me out and explain why? Thanks.
    In the second one, multiply the fraction by \frac{\sqrt3}{\sqrt3}
    Offline

    12
    ReputationRep:
    (Original post by RDKGames)
    In the second one, multiply the fraction by \frac{\sqrt3}{\sqrt3}
    Probably easier to rewrite  \sqrt{27} as  \frac{9}{\sqrt{3}} and then rationalise.
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by NeverLucky)
    Probably easier to rewrite  \sqrt{27} as  \frac{9}{\sqrt{3}} and then rationalise.
    I'd find it easier to rewrite it as 3\sqrt3 then add on 2\sqrt3. Each to their own I guess.
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by NeverLucky)
    Can you see what would happen if you multiplied top and bottom by 1 - \frac{1}{\sqrt{2}}?
    oh wow xD
    So you're saying the denominator here is
    1 + \frac{1}{\sqrt{2}}? and not sqrt 2?
    • Thread Starter
    Offline

    12
    ReputationRep:
    Maybe I should ask a better question: why so I add one to the fraction instead of rationalise it first? (at least thats what the mark scheme says)
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by Gabzinc)
    Maybe I should ask a better question: why so I add one to the fraction instead of rationalise it first? (at least thats what the mark scheme says)
    I didn't understand either of your questions...
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by RDKGames)
    I didn't understand either of your questions...
    Sorry, I'm being really confusing :s

    In the first question,

    Usually the first thing I would do is rationalize the 1/√2. Apparently this is wrong, and I am supposed to join the 1 and the 1/√2 first. Just want to know why.

    In the second question,



    I tried to add them like I would ordinary fractions. Obviously, this was wrong as well.

    In a nutshell, I just want to know when exactly to rationalize the denominator, as I'm clearly doing it wrong :/
    Offline

    12
    ReputationRep:
    (Original post by Gabzinc)
    Maybe I should ask a better question: why so I add one to the fraction instead of rationalise it first? (at least thats what the mark scheme says)
    You can rationalise it first or you can add up the bottom part of the fraction first. It doesn't matter which way you do it - you'll get the same answer either way.
    • Thread Starter
    Offline

    12
    ReputationRep:
    (Original post by NeverLucky)
    You can rationalise it first or you can add up the bottom part of the fraction first. It doesn't matter which way you do it - you'll get the same answer either way.
    Oh... just tried it and it worked, thanks! I think I was rationing incorrectly all this time...
    oops.

    That's really embarrassing.

    Thanks for the help, and sorry for wasting your time !
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: August 27, 2016
  • create my feed
  • edit my feed
Poll
Do you agree with the proposed ban on plastic straws and cotton buds?
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.