IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Math => Topic started by: Cantwaittillitsover! on May 11, 2010, 07:16:34 am

Title: MATH P1 question.
Post by: Cantwaittillitsover! on May 11, 2010, 07:16:34 am: Hey guys.

This question is from Nov 2009 P1 paper.

3) (i) Find the first 3 terms in the expansion of (2-x)^6 in ascending powers of x.

(ii) Given the coefficient of x^2 in the expansion of (1+2x+ax^2)(2-x)^6 is 48, find the value of the constant a.

My answer for part (i) is:
64-192x+240x^2

i need help with part (ii)

Thanks.
Title: Re: MATH P1 question.
Post by: nid404 on May 11, 2010, 07:36:03 am: I just finished this paper :)

So basically try to think of all possible combinations that would give terms with $x^2$

From the first part that you calculated
$64-192x+240x^2$

if you multiply this term with $(1+2x+ax^2)$

possible terms with $x^2$ would be

$64X ax^2$ , $-192x X 2x$ , & $240x^2 X 1$

The sum of their co-efficient = 48 given in the question

64a- (192X2) + 240=48

64a= 48-240+(192X2)
64a= 192
a=192/64
$a=3$
Title: Re: MATH P1 question.
Post by: nipuna92 on May 11, 2010, 07:37:02 am: Okay this is easy

(2-x)6 = 64-192x + 240x^2

now

(1 + 2x + ax^2)(64-192x + 240x^2)
Expand the ones u need only (x^2)

(2x into -192x) + (240x^2) + (64ax^2) =48
take the coeffcients,

-384 + 240 + 64a =48
-144 + 64a =48
64a = 48 + 144
a= 192 / 64
a = 3

Practice past paers then ull get it
good luck
Title: Re: MATH P1 question.
Post by: nipuna92 on May 11, 2010, 07:38:38 am: oops i didnt know that u had posted the answer
sorry nid404
Title: Re: MATH P1 question.
Post by: nid404 on May 11, 2010, 07:39:52 am: lol..no problem
Glad to see you helping :)