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 Posted: Mon Jun 13, 2016 7:34 pm

Joined: Tue Oct 02, 2007 10:58 am
Posts: 132
I've told DD to ignore this one if it comes up as (again) neither of us can work it out from the mark shceme.

But G55 if you're around, or anyone else Mathsy, it would be great to know how to do this. DD has looked at her power rules but gets 1/25 whereas the mark scheme says the answer is 2.

The question is (where ^ is to the power of as you used previously):

5^x = 125 ^ -2/3 (that's 125 to the power of minus two-thirds)

What is the value of x?

It's from Jun 2012 edexcel linked pair pilot Methods 1 (higher).

Thank you

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 Posted: Mon Jun 13, 2016 7:49 pm

Joined: Tue Apr 08, 2014 9:25 pm
Posts: 245
I am no maths teacher, but my 2 cents are that your DD seems to be solving it completely instead of leaving her calculation as a power of 5. The q wants you to find the value of the power of 5 when you solve everything on the right side of the equation.

Also I think the answer should be x=-2 not x=2.

5^x = 125^ -2/3 = 1/125^(2/3) = 1/(125^1/3)^2=1/5^2=5^ -2

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 Posted: Mon Jun 13, 2016 8:01 pm

Joined: Tue Oct 02, 2007 10:58 am
Posts: 132
Thanks mumsdarling, you're quite right, we both missed that. And it is -2 not 2.

Must remember to RTQ and ATQ!

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 Posted: Mon Jun 13, 2016 8:17 pm

Joined: Tue Apr 08, 2014 9:25 pm
Posts: 245
You are welcome. Good luck for your DDs exam.

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 Posted: Mon Jun 13, 2016 9:25 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 16133
Sorry - been out - shocking I know!

125^ (-2/3) the negative means take the reciprocal ie 1 over the fraction.
Then the denominator represents 'root' so we want the cube root of 125 which is 5 then we have our power of 5 as required.

If you have 8^(2/3) it's usually easier to take the root befoor taking the power so I'd cube root 8 to get 2 and then square that ie answer is 4.

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 Posted: Tue Jun 14, 2016 8:46 am

Joined: Fri Oct 11, 2013 8:55 am
Posts: 500
I think the simplest way to solve it is as follows:

First hopefully they should recognise that 125 = 5 x 5 x 5 = 5^3
Also when you have a power of a power you just multiply them together. e.g. (10^2)^3 = 10^(2x3) = 10^6

So,

125^(-2/3) = (5^3)^(-2/3) = 5^(3 x -2/3) = 5^(-2)

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 Posted: Tue Jun 14, 2016 3:46 pm

Joined: Sat Jul 24, 2010 9:45 pm
Posts: 1586
I like that. I always tell my DS to simplify all he can in his calculations.

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It felt like I hit rock bottom; suddenly, there was knocking from beneath... (anon.)

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 Posted: Tue Jun 14, 2016 7:49 pm

Joined: Tue Oct 02, 2007 10:58 am
Posts: 132
Thank you G55, proud_dad and mumsd'g- DD is shattered- she had 2 GCSEs today, Maths this morning and History this afternoon, then it's double Chemistry tomorrow so I haven't asked whether this type came up today.

Her last Maths paper is on Thursday so no doubt she'll have a last minute question I can't answer and may need your help with!
Neither of us is what you would call a natural mathematician so, like you purpleduck, I agree that simple is definitely good. Mind you we both enjoy persevering with a problem and Maths has been a great way for DD to discover that determination to work through things that may not come easily first time does pay off.
I am slightly at all the Maths sets at her school following the double GCSE. It takes up a lot of time and the additional two Maths exams this week are making it a very tiring week for DD. It's great for those who end up with 2 good grades but perhaps not so much for those who struggle.

I'm guessing that some of the additional material will have found its way into the new 9-1 Maths GCSE.

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 Posted: Tue Jun 14, 2016 8:53 pm

Joined: Sat Jul 24, 2010 9:45 pm
Posts: 1586
Kit - are you sure your DD will get two separate maths grades? DS had two maths papers, too, but his final GCSE maths grade will be an average of the two. Or maybe different exam boards have different ways to do it?

_________________
It felt like I hit rock bottom; suddenly, there was knocking from beneath... (anon.)

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 Posted: Tue Jun 14, 2016 8:54 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 16133
I promise not to go out tomorrow evening

I like the Twin Award - it has some interesting questions - it's better than Add Maths I think.

Answering the above post - the Twin award has four papers and they get two GCSEs - methods and applications.

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