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# Binomial Expansion watch

1. I'm doing my A levels and really stuck on this question. I came across it on a past paper.

The expansion of (1-x)(a+bx)^3 in ascending powers of x begins 8 - 28x....
Find the values of a and b.

I have already expanded (a+bx)^3 to give a³ + 3a²bx + 3ab²x² + b³x³

How do I do the second part - finding a and b?
2. OK - you've done the first bit.

Now you can multiply what you've got by the (1-x) so you get the series for the whole thing.

Then you look at the coefficients. How many units have you got? a^3. How many are you told that you have? 8. So you have a.

Then you look at how many x's you have. You have a already. So figure out b.

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Updated: February 3, 2010
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