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Rationalise the denominator – GCSE maths grade 6+

November 4, 2015 by Simon 16 Comments

This video is about how to ‘rationalise the denominator’ – which is just a way of changing the bottom of a fraction to a whole number – and gives a few examples:

5 divided by 3√2 becomes 5√2 / 6

and

18 divided by √3 becomes 6√3

Like many of these exercises, in isolation, they appear to be fairly irrelevant – it’s only when you need to manipulate equations in higher levels, that they can be useful. Most ‘rationalise the denominator’ questions appear on non calculator GCSE maths papers (including Edexcel, AQA, OCR and Educas), and are approximately grade 6+. The questions are usually 2 – 4 marks.

Please do try some of the questions in the additional posts listed below – with a little practice learning how to rationalise the denominator can be fairly straightforward.

This series of 3 minute math is designed as a quick reminder for some of the main topics. I hope they help to provide a focus and a way forward. If you need any more detail please search the site or contact me – always pleased to answer any questions!

Here’s some additional posts, that might be of interest:

Working with surds

Surds ‘show that’ questions

Watch the video on YouTube

 

Comments

  1. Julius Lucky says

    December 6, 2014 at 1:19 pm

    Reply
  2. Julius Lucky says

    December 6, 2014 at 1:18 pm

    THANK YOU so much Mr Simon. Took me less than 3 minutes to understand what
    my teacher took me 30minutes to teach me!

    Reply
  3. The Universe Of Hannah says

    October 18, 2014 at 12:04 pm

    Thank you this was really helpful, but what do i do when it’s: 6+ root 8

    root 2 ?

    Reply
  4. Gyan Chappory says

    April 2, 2014 at 5:13 pm

    how can u solve.. 3 root 7 + 3 root 7??

    Reply
  5. Iuliana Costache says

    December 11, 2013 at 8:59 pm

    If only all of the teachers would explain maths the way you do, sir! 🙂
    Thank you for posting all of those helpful videos.

    Reply
  6. Simon Deacon says

    December 8, 2013 at 9:42 am

    Please like and leave a comment!

    Visit https://www.3minutemaths.co.uk for real maths, tips and techniques.

    Reply
  7. Simon Deacon says

    November 6, 2013 at 8:44 pm

    Will do – I’ll let you know when posted.

    Reply
  8. Alex Goody says

    November 6, 2013 at 8:37 pm

    Would it be possible to do a video on Set Theory (Venn Diagrams), if so
    that would be very useful!

    Reply
  9. Simon Deacon says

    November 4, 2013 at 10:10 am

    Thanks and glad it helped 🙂

    Reply
  10. Alex Goody says

    November 3, 2013 at 10:57 pm

    Very helpful, keep up the good work !!

    Reply
  11. Simon Deacon says

    November 3, 2013 at 9:46 pm

    Hi – try ‘3 Minute Math – Angles and Number of Sides in Regular Polygons 2’
    or ‘3 Minute Math – Angles and Number of Sides in Regular Polygons.’ I hope
    these help and all best S

    Reply
  12. Simon Deacon says

    November 3, 2013 at 9:40 pm

    Hi Will – you need to multiply top and bottom by root6. That will give you
    3 root6 on the top and 6 on the bottom. If you then divide top and bottom
    by 3 you get root6 / 2 Hope this helps and all best S

    Reply
  13. Will Jolliffe says

    November 3, 2013 at 9:19 pm

    Hi ive been asked to rationalise the denominator of 3/root6… the answer
    according to the mark scheme is root6/2. Could you tell me how to get to
    that answer? Thanks

    Reply
  14. Simon Deacon says

    November 3, 2013 at 4:50 pm

    Will do and will let you know when posted. All the exterior angles of a
    regular polygon add up to 360 degrees… the interiors can be calculated by
    180 – exterior. Hope this helps and thanks for the request. All best S

    Reply
  15. Forheloveoflucie21 says

    November 3, 2013 at 3:17 pm

    Please can you do a video on finding interior and extirior angles in
    regular and irregular polygons?

    Reply
  16. Simon Deacon says

    November 3, 2013 at 10:30 am

    Reply

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