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# Undoing binomial expansion watch

1. Hello everyone,

I don't know how to start this question:

"When (1+ax)^n is expanded in ascending powers of x, the first three terms are 1 - 28x + 294x^2. Find a and n"

Am I right in saying n = 4?
2. (Original post by Electrogeek)
Hello everyone,

I don't know how to start this question:

"When (1+ax)^n is expanded in ascending powers of x, the first three terms are 1 - 28x + 294x^2. Find a and n"

Am I right in saying n = -4?
Do you know the general formula for the expansion of (1+x)^n? It's on the (Edexcel) formula book.

The expansion of (1+ax)^n is the same but with ax instead of x.

You can compare coefficients once you have the general formula.
3. You can only find the original expression if the series is finite i.e. when the exponent is a positive integer. In that case, finding the exponent just requires that you take two of the coefficients and form a linear system which you can then solve.
4. (Original post by VannR)
You can only find the original expression if the series is finite i.e. when the exponent is a positive integer. In that case, finding the exponent just requires that you take two of the coefficients and form a linear system which you can then solve.
I managed to do it via simultaneous equations for both variables. Thanks for the help though.
5. (Original post by Electrogeek)
I managed to do it via simultaneous equations for both variables. Thanks for the help though.
That's what a linear system is......
6. (Original post by VannR)
That's what a linear system is......
Oh... Didn't know that... Thanks anyway.

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