MATH P1 question.

[Cantwaittillitsover!]

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.

[nid404]

I just finished this paper 

So basically try to think of all possible combinations that would give terms with

From the first part that you calculated


if you multiply this term with 

possible terms with   would be

, , &

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

64a- (192X2) + 240=48
 
64a= 48-240+(192X2)
64a= 192
a=192/64

[nipuna92]

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

[nipuna92]

oops i didnt know that u had posted the answer
sorry nid404

[nid404]

lol..no problem
Glad to see you helping