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C4 quick question

Question:

Find the binomial expansion up to x^2 of sqrt(1-2x). By substituting x=0.05 find an approximation to sqrt(10).

I know the first part, i get 3 terms, but the second part, when I substitute x=0.05 the expansion becomes sqrt(0.9). How can we do this question?

and also, there is one more question, it says the coefficient of x^2 in the expansion sqrt(4+bx) is -9. find the value of b and the corresponding coefficient of x. does this mean the coefficinet of x^3?

thank you!
Reply 1
Avatar for vahik92
vahik92
OP
1
anyone??
Reply 2
Avatar for kosy91
kosy91
not x^3...they want coefficient of x
Reply 3
Avatar for steve2005
steve2005
17
vahik92
Question:

Find the binomial expansion up to x^2 of sqrt(1-2x). By substituting x=0.05 find an approximation to sqrt(10).

I know the first part, i get 3 terms, but the second part, when I substitute x=0.05 the expansion becomes sqrt(0.9). How can we do this question?

and also, there is one more question, it says the coefficient of x^2 in the expansion sqrt(4+bx) is -9. find the value of b and the corresponding coefficient of x. does this mean the coefficinet of x^3?

thank you!


Are you sure its 0.05 and not 0.005
Reply 4
Avatar for vahik92
vahik92
OP
1
kosy91
not x^3...they want coefficient of x


oh ok :smile:
Reply 5
Avatar for vahik92
vahik92
OP
1
steve2005
Are you sure its 0.05 and not 0.005


even if it is 0.005, it still doesn't give 10 in the square root does it? i dont have the book right now with me, but even if you know the answer with x=0.005 i wud still get the concept...
Reply 6
Avatar for vahik92
vahik92
OP
1
anyone????????
Reply 7
Avatar for steve2005
steve2005
17
vahik92
anyone????????


I agree it is a puzzle. Where does the question come from? If an old paper, which one?

Edit: I have just noticed its from a book. Which one ?
Reply 8
Avatar for TwinPoker
TwinPoker
I'm pretty sure it would be 0.005 not 0.05 because of the vaild ranges of x for the equation to work.
Reply 9
Avatar for azamally
azamally
vahik92
anyone????????


You can write the sqrt(0.9) as [sqrt9/sqrt10], then transpose to solve for sqrt10
azamally
You can write the sqrt(0.9) as [sqrt9/sqrt10], then transpose to solve for sqrt10


Yes, very clever.

But dividing 3 by 0.94875 needs a calculator so one may as well use the calculator to find the square root of ten.

Is there an easier calculation?
Reply 11
Avatar for azamally
azamally
steve2005
Yes, very clever.

But dividing 3 by 0.94875 needs a calculator so one may as well use the calculator to find the square root of ten.

Is there an easier calculation?


That's the way I learnt it this year... I guess the examiners just need you to show the steps
azamally
You can write the sqrt(0.9) as [sqrt9/sqrt10], then transpose to solve for sqrt10


this is the answer, doesnt matter if you use ur calc, the examiner just wants to see you show the steps and show how you got ur answer. using the expansion, ur answer will be different from if you just punched in >root<10 on ur calc, thats why the questions usually say put ur answer in something unusual like 7 decimal places...

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