The Student Room Group
Reply 1
What is the general formula as you understand it?
Reply 2
pls 4give me i duno how to use latex but here we go:

(a+b)^n = a^n + (nchoose0)a^n + (nchoose1)a^n-1 X b + (nchoose2)a^n-2 X b^2

sorry tht looks sooo confusin bt hopefully u understnd
Reply 3
Not quite.

(a+b)n=(n0)an+(n1)an1b+(n2)an2b2++(nn)bn\displaystyle (a + b)^n = \begin{pmatrix} n \\ 0 \end{pmatrix} a^n + \begin{pmatrix} n \\ 1 \end{pmatrix} a^{n-1} b + \begin{pmatrix} n \\ 2 \end{pmatrix} a^{n-2} b^2 + \cdots + \begin{pmatrix} n \\ n \end{pmatrix} b^n

So you have an extra ana^n at the beginning. However, (n0)=(nn)=1\displaystyle \begin{pmatrix} n \\ 0 \end{pmatrix} = \begin{pmatrix} n \\ n \end{pmatrix} = 1, so you can just drop the coefficients of ana^n and bnb^n if you find it easier to remember that way.

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