rationalise the denominator

mufc802 · 13 Apr 2011 at 23:37

Probably missing something obvious but..

Q. "root3 / root15"

I got it to "root45 / 15" but how do I get that to "root5 / 5" which it says on the calculator?

nydryl · 13 Apr 2011 at 23:59

sqrt(15) = sqrt(3)sqrt(5)

Cancel the sqrt(3) so you have 1/sqrt(5) which is the same as sqrt(5)/5 as you can write 5 as sqrt(5)sqrt(5) and cancel a sqrt(5).

mufc802 · 14 Apr 2011 at 00:06

Ah I get it now, I tried multiplying the denominator first which I could have done after simplifying it.

Jono · 14 Apr 2011 at 00:09

Having not done any real maths I fully understood since I did my GCSEs - I did a whole bunch of Calculus stuff at uni, but although I scored 100% on one of the exams, i really don't understand it now (something like Laplace Transforms - is there anywhere for adults who should be less dim to re-get to grips with maths and stuff online?

IceBus · 14 Apr 2011 at 09:00

www.mathdvdtutor.com - I really like this guy. You can watch a bunch free too.

mufc802 · 14 Apr 2011 at 23:25

How do you answer: (5 root 2)(root 8+root 7)?

Calculator says 20+5root14.

Nitefly · 14 Apr 2011 at 23:30

This is becoming your home work forum isn't it?

Not that I mind, I've refreshed my memory for a few things

Hxc · 14 Apr 2011 at 23:32

Multiply by root15.

mufc802 · 14 Apr 2011 at 23:50

Nitefly said:
This is becoming your home work forum isn't it?

Not that I mind, I've refreshed my memory for a few things

Since I'm teaching myself I have no one to help so have to ask here.

Nitefly · 14 Apr 2011 at 23:52

mufc802 said:
Since I'm teaching myself I have no one to help so have to ask here.

I see, what is the qualification in aid of (what are you aiming for)?

Regardless, good effort, best of luck!

mufc802 · 15 Apr 2011 at 00:01

Nitefly said:
I see, what is the qualification in aid of (what are you aiming for)?

Regardless, good effort, best of luck!

A-Level maths because I hear it's very important in an engineering degree.

TheCenturion · 15 Apr 2011 at 01:35

khanacademy.org

Redmint · 15 Apr 2011 at 01:48

mufc802 said:
A-Level maths because I hear it's very important in an engineering degree.

I did electronic eng and I can say you will use a fair bit of the A-level stuff.

Mason- · 15 Apr 2011 at 02:50

mufc802 said:
How do you answer: (5 root 2)(root 8+root 7)?

Calculator says 20+5root14.

5 = sqrt 25
5 sqrt 2 = sqrt 50

sqrt50*sqrt8 + sqrt50*sqrt7
=
sqrt400 + sqrt350
sqrt 400 = 20
20 + sqrt350
sqrt350 = 5*sqrt14

so

20 + 5sqrt14

Also what you did on the first question is fine.
sqrt 3 / sqrt 15
=
sqrt 45 / 15
sqrt 45 = 3sqrt5
3sqrt5 / 15

cancel out factor of 3

sqrt 5 / 5