Author Topic: Binomial Expansion Question  (Read 778 times)

Offline Dania

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Binomial Expansion Question
« 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.
:)

Offline SpongeBob

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Re: Binomial Expansion Question
« Reply #1 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


« Last Edit: September 30, 2010, 04:43:48 pm by SpongeBob »
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Offline S.M.A.T

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Re: Binomial Expansion Question
« Reply #2 on: September 30, 2010, 05:07:32 pm »
thanks +rep :D


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