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Undoing binomial expansion Watch

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    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?
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    (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.
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    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.
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    (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.
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    (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......
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    (Original post by VannR)
    That's what a linear system is......
    Oh... Didn't know that... Thanks anyway.
 
 
 
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