You are Here: Home >< Maths

# C4 binomial theorem watch

1. part a) find binomial expansion of (1+2x)^(-1/2)
part b) using part a show with x=-0.1 sqr(5)=2.23
by substituting in -0.1 and multiplying by 2 i only get 2.215

What have i done wrong?
2. (Original post by TheGreatPumpkin)
part a) find binomial expansion of (1+2x)^(1/2)
part b) using part a show with x=-0.1 sqr(5)=2.23
by substituting in -0.1 and multiplying by 2 i only get 2.215

What have i done wrong?

It should be 1 + x - x^2 / 2.

Check it again.
3. (Original post by Zacken)

It should be 1 + x - x^2 / 2.

Check it again.
Sorry I made a mistake typing it was (1+2x)^-(1/2) I've changed it now
4. (Original post by TheGreatPumpkin)
Sorry I made a mistake typing it was (1+2x)^-(1/2) I've changed it now
Your expansion is still wrong. It should be
5. (Original post by Zacken)
Your expansion is still wrong. It should be
I don't understand, to work out the co-efficient of x^2 don't you (-0.5)(-1.5)(2)/2 which gives 3/4,
like the formula n(n-1)/2
6. (Original post by TheGreatPumpkin)
I don't understand, to work out the co-efficient of x^2 don't you (-0.5)(-1.5)(2)/2 which gives 3/4,
like the formula n(n-1)/2
You might want to find the expansion of and then replace y by 2x...
7. (Original post by DFranklin)
You might want to find the expansion of and then replace y by 2x...
That's just 1 - x/2 + 3x^2 /8
1/2^2= 1/4
1/4 +1/2 is 3/4
(3/4)/2=3/8
8. the coefficient of x is also squared in the x^2 term
9. (Original post by TheGreatPumpkin)
I don't understand, to work out the co-efficient of x^2 don't you (-0.5)(-1.5)(2)/2 which gives 3/4,
like the formula n(n-1)/2
As the above posts suggest, it'll be (-0.5)(-1.5)(2)/2 * (2x)^2
10. (Original post by Andrew Dainty)
the coefficient of x is also squared in the x^2 term
Oh thank you

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 21, 2017
Today on TSR

### University open days

Wed, 21 Nov '18
• Buckinghamshire New University
Wed, 21 Nov '18
• Heriot-Watt University
Wed, 21 Nov '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams