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

Qualification => Subject Doubts => GCE AS & A2 Level => Math => Topic started by: Dania on September 30, 2010, 04:02:51 pm

Title: Binomial Expansion Question
Post by: Dania on September 30, 2010, 04:02:51 pm

I always seem to have problems solving questions such as (ii) below & I've seen them in pretty much every past paper. Can someone please explain to me how to solve (ii)?

(i) Find the first 3 terms in the expansion of (2 + 3x)^5 in ascending powers of x.
(ii) Hence find the value of the constant a for which there is no term in x^2 in the expansion of
(1 + ax)(2 + 3x)^2.

Thanks.

Title: Re: Binomial Expansion Question
Post by: SpongeBob on September 30, 2010, 04:24:06 pm

(i) 32+ 240x+720x²

(ii)(1 + ax)(2 + 3x)^2  

I think you meant 5?

(1+ax)(32+ 240x+720x²)

32+210x+720x²+ 32ax+240ax²+720ax³

There is no term in x²
So 720x²+240ax²=0

a=-3

Title: Re: Binomial Expansion Question
Post by: S.M.A.T on September 30, 2010, 05:07:32 pm

thanks +rep :D