# A2 binomial expansion help

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#1
For part b below, I plugged 1/8 into root[(1-5x)/(1+x)] which gave root3/3

So i then plugged 1/8 into my estimation expansion of 1-4x-3x²/2 , and multiplied the result by 3
(As you x3 to go from root3/3 to root3), giving 231/128.
However the MS says the answer's 128/77 - what have i done wrong? Thanks

Last edited by Rhys_M; 1 month ago
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1 month ago
#2
What would you get if, instead of multiplying by 3 on both sides, you invert the fractions on both sides?
1
#3
(Original post by FlatearthFTW)
What would you get if, instead of multiplying by 3 on both sides, you invert the fractions on both sides?
Then it'd be 3/root3 and 1/root3.
How does that help?
0
1 month ago
#4
(Original post by Rhys_M)
Then it'd be 3/root3 and 1/root3.
How does that help?
Substituting x=1/8 will give you

for some p,q that you've determined.

0
1 month ago
#5
(Original post by Rhys_M)
Then it'd be 3/root3 and 1/root3.
How does that help?
Two ways of looking at it.

or

0
#6
(Original post by Plücker)
Two ways of looking at it.

or

(Original post by ghostwalker)
Substituting x=1/8 will give you

for some p,q that you've determined.

(Original post by Rhys_M)
Then it'd be 3/root3 and 1/root3.
How does that help?
Thanks so much everyone
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