# Dr Frost A level MathsWatch

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#1
Last edited by CardiBSuperfan3; 5 months ago
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5 months ago
#2
(Original post by MutsaMun)
/Users/Mutsa/Desktop/Screenshot 2019-09-04 at 17.01.29.png
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#3
just managed to figure out how to attach it
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#4
(Original post by RDKGames)
here
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5 months ago
#5
(Original post by CardiBSuperfan3)
just managed to figure out how to attach it
Expanding in ascending powers of is the same as expanding in ascending powers of which is the same as expanding each term individually in ascending powers of and adding the common terms together.

So, since we are interested in powers up to and including , you should expand and individually first.
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5 months ago
#6
(Original post by CardiBSuperfan3)
use binomial expansion. i dont know what examboard you do but the formula is in the edexcel formula booklet. the 1/(x-1) would become (x-1)^-1 coz its a reciprocal. than you just use the formula and write down all the values up to x^2.
hope that helped if you cant figure out still let me know ill give it a try myself
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#7
hi yeah im doing edexcel
i can get up to doing the reciprocals but i dont know where to go from there
Last edited by CardiBSuperfan3; 5 months ago
0
5 months ago
#8
(Original post by CardiBSuperfan3)
hi yeah im doing edexcel
i can get up to doing the reciprocals but i dont know where to go from there
Are you familiar with the binomial expansion formula

??
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5 months ago
#9
(Original post by CardiBSuperfan3)
hi yeah im doing edexcel
i can get up to doing the reciprocals but i dont know where to go from there

ive got
2 - (x-1)^-1 4(x 2)^-1
ill give it a try and let you know in a bit...xxx
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#10
(Original post by RDKGames)
Are you familiar with the binomial expansion formula

??
yes
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5 months ago
#11
(Original post by CardiBSuperfan3)
yes
So use that.

Notice that the middle term can be rewritten;

where the difference between this and is that you firstly replace in the formula I gave you by and set . Get the terms up to . What do you get?

Repeat for which requires a slight modification before the formula above can be applied in the same way.
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#12
(Original post by RDKGames)
So use that.

Notice that the middle term can be rewritten;

where the difference between this and is that you firstly replace in the formula I gave you by and set . Get the terms up to . What do you get?

Repeat for which requires a slight modification before the formula above can be applied in the same way.
okay for the first bit i got 1 + x + x^2
and then i got 8 - 4x - x^2
which I'm pretty sure is completely wrong because it said i would somehow have fractions
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5 months ago
#13
(Original post by CardiBSuperfan3)
okay for the first bit i got 1 + x + x^2
and then i got 8 - 4x - x^2
which I'm pretty sure is completely wrong because it said i would somehow have fractions
Yep it's wrong.

The first bit looks good though, it's .

Then for the second bit, you need to realise that to expand using the binomial expansion I posted above, you first need to make sure that rather than being 2+... it's actually 1+... inside the bracket.
How do we do that? Simply factor out a 2 out of the bracket. Then it becomes .
So we reduce the problem to

Now you need to expand just using the binomial formula above. Simply replace by and set . Expand up to and then multiply all coefficients by 2 since we seek .
1
#14
(Original post by RDKGames)
Yep it's wrong.

The first bit looks good though, it's .

Then for the second bit, you need to realise that to expand using the binomial expansion I posted above, you first need to make sure that rather than being 2+... it's actually 1+... inside the bracket.
How do we do that? Simply factor out a 2 out of the bracket. Then it becomes .
So we reduce the problem to

Now you need to expand just using the binomial formula above. Simply replace by and set . Expand up to and then multiply all coefficients by 2 since we seek .
yeah that's what i did but do i not need to then multiply by 4?
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5 months ago
#15
(Original post by CardiBSuperfan3)
yeah that's what i did but do i not need to then multiply by 4?
I've shown you that is the same as .

So when you expand you need to multiply by 2 since that's what's in front of it, not 4.
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