Surds: Dividing by Negative Numbers

\dfrac{1-\sqrt{2}}{-1} = \sqrt{2}-1

Reply 2

OP

Original post by TenOfThem

\dfrac{1-\sqrt{2}}{-1} = \sqrt{2}-1

Thanks, but how do you get to that answer?

Reply 3

steve2005

17

Original post by TheWantedGuy

Hey,

Just wondering if anyone could help with some surd questions which I'm having some difficulty with, at the moment.

So the question is to rationalise:

1/1+(sq root of 2); so far I've gotten:

1(1-(sq root of 2)/-1... and I'm not sure how you deal with this negative number on the bottom, because the answer doesn't include a negative number in the bottom.

What do the coloured brackets do?
Look what happens when the brackets are removed.

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Reply 4

OP

Original post by steve2005

What do the coloured brackets do?
Look what happens when the brackets are removed.

Uploaded with ImageShack.us

Sorry, I'm not too sure what you're asking me to do...

Original post by TheWantedGuy

Thanks, but how do you get to that answer?

Dividing by -1 is exactly the same as multiplying by -1.

Reply 6

What is 3 divided by -1

What is -7 divided by -1

Reply 7

OP

Original post by Mr M

Dividing by -1 is exactly the same as multiplying by -1.

Oh okay, thanks. But that only works for -1, right? So, in another example:

2/1+(sq root of 6)... I've got:

2*1-(sq root of 6)/-5, how do I make the -5 positive?

Reply 8

OP

Original post by TenOfThem

What is 3 divided by -1

What is -7 divided by -1

-3, -7... I presume, you're trying to say /-1 = *-1, right? But how about the example I've just posted?

Original post by TheWantedGuy

Oh okay, thanks. But that only works for -1, right? So, in another example:

2/1+(sq root of 6)... I've got:

2*1-(sq root of 6)/-5, how do I make the -5 positive?

Divide by 5 and then multiply by -1?

Or divide by 5 and change the signs?

Reply 10

You multiply everything by -1

Actually would it be churlish just to say divide each term by -5?

Reply 12

OP

Original post by Mr M

Divide by 5 and then multiply by -1?

Or divide by 5 and change the signs?

Hmm, I'm not sure how I'd do that without a calculator.

Reply 13

OP

Original post by TenOfThem

You multiply everything by -1

Sorry, I meant this one:

"2/1+(sq root of 6)... I've got:

2*1-(sq root of 6)/-5, how do I make the -5 positive? "

Original post by TheWantedGuy

Hmm, I'm not sure how I'd do that without a calculator.

Seriously? You don't know what 2/5 and 6/5 are as decimals?

Anyway there is no need to change it into a decimal - leave it as a fraction, just change the signs and leave the 5 on the bottom.

Reply 15

Original post by TheWantedGuy

Sorry, I meant this one:

"2/1+(sq root of 6)... I've got:

2*1-(sq root of 6)/-5, how do I make the -5 positive? "

So did I

Reply 16

OP

Original post by Mr M

Seriously? You don't know what 2/5 and 6/5 are as decimals?

Anyway there is no need to change it into a decimal - leave it as a fraction, just change the signs and leave the 5 on the bottom.

Lol yeah I can do it, I just don't think that's what you're meant to do... so how are you suggesting I work it out?

Original post by TheWantedGuy

Sorry, I meant this one:

"2/1+(sq root of 6)... I've got:

2*1-(sq root of 6)/-5, how do I make the -5 positive? "

That's wrong anyway.

You should have:

\frac{2-2\sqrt6}{-5}

I suspect you are not rationalising the denominator correctly as you did not recognise what steve told you earlier.

Reply 18

Original post by TheWantedGuy

Lol yeah I can do it, I just don't think that's what you're meant to do... so how are you suggesting I work it out?

Seriously

What is 3 divided by -1
Or -7 divided by -1

do you not know?

Reply 19

OP