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Surds A level maths question? - Hello - 02-19-2014 12:45 PM
I'm revising for my A levels, and was doing fine until i came across this question. It was to rationalise the denominator, simplifying where possible.
I uploaded a photo of what i did to a very very old twitter account of mine, if anyone could have a look? The answer underlined is what i got, however the one circled is what the actual answer is. Can someone explain to me what i've done wrong and how i would get the right answer?
http://twitter.com/HelloJessieJ/status/419845879949189120/photo/1
- Jenn - 02-19-2014 12:51 PM
(Squareroot 3 + 7)*(squareroot 3 - 7) should give you (squareroot 3 - 7)^2= 3-(14*squareroot 3)+49
- abhi - 02-19-2014 12:59 PM
actually u did a little mistake while rationalising denominator.
first tell me is it 3 root 3 or cube root of 3???
- Archimedes - 02-19-2014 01:00 PM
You used an incorrect rationalizing fraction...
your denominator is: (3√3 + 7)
so you should multiply numerator and denominator by the conjugate which is: (3√3 - 7)
to get...
[11(3√3 - 7)]/(27 - 49) = [- 11(7 - 3√3)]/(- 22) = (7 - 3√3)/2
archimedes
[the way you write radical expressions is WRONG ! ]
- M@thM@ni@c - 02-19-2014 01:05 PM
I would point out the same as Abhi...your original problem at the beginning is not the one you did your solution for. You dropped the 3; whether it is a cubed root or a factor would make a huge difference.
- David - 02-19-2014 01:09 PM
As some others already pointed out. The original question is:
11 / (3√3 + 7), but then the first 3 somehow got dropped out.
So your √9, should've been 9√9 = 27, then you get the correct answer.