AQA core 4 exam paper - binomial question

Really having trouble getting my head around the last section of this question and the mark scheme really isn't illuminating anything so perhaps you could help me.
the question goes like so
b) obtain binomial expansion of 1/root(1+2x)
which i have correctly done and have obtained 1-x + 3/2(x²)
C) use your answer to part b with x = -0.1 to show that rt5 is approx 2.23
this is where i am confused, obviously to start put the x value into my expansion which is correct but then couldn't work out how that possibly translates into a root 5 approximation. Can anyone help?

Reply 1

generalebriety

16

x = -0.1:
1/root(1+2x) = 1/root(0.8) = 1/root(4/5) = root 5 / root 4 = 1/2 root 5. :smile:

Reply 2

steve2005

17

Plummie

Really having trouble getting my head around the last section of this question and the mark scheme really isn't illuminating anything so perhaps you could help me.
the question goes like so
b) obtain binomial expansion of 1/root(1+2x)
which i have correctly done and have obtained 1-x + 3/2(x²)
C) use your answer to part b with x = -0.1 to show that rt5 is approx 2.23
this is where i am confused, obviously to start put the x value into my expansion which is correct but then couldn't work out how that possibly translates into a root 5 approximation. Can anyone help?

Here

Picture 224.png37.7KB

Reply 3

Plummie

OP

thankyou, i see the connection now, shall have to think better in future,

AQA core 4 exam paper - binomial question

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