General Term in Binomial Expansion

Here you will learn formula to find the general term in binomial expansion with examples.

Let’s begin –

General Term in Binomial Expansion

We have,

\((x + a)^n\) = \(^{n}C_0 x^n a^0\) + \(^{n}C_1 x^{n – 1} a^1\) + …………… + \(^{n}C_r x^{n – r} a^r\) + …………… + \(^{n}C_n x^0 a^n\)

We find that : The first term = \(^{n}C_0 x^n a^0\)

The second term = \(^{n}C_1 x^{n – 1} a^1\)

The third term = \(^{n}C_2 x^{n – 2} a^2\)

The fourth term = \(^{n}C_3 x^{n – 3} a^3\),  and so on.

We thus observe that the suffix of C in any term is one less than the number of terms, the index of x is n minus the suffix of C and the index of a is the same as the suffix of C.

Hence, the (r + 1)th term is given by \(^{n}C_r x^{n – r} a^r\)

Thus, if \(T_{r + 1}\) denotes the (r + 1)th term, then

General Term :

\(T_{r + 1}\) = \(^{n}C_r x^{n – r} a^r\)

This is called the general term, because by giving different values to r we can determine all terms of the expansion.

In the binomial expansion of \((x – a)^n\), the general term is given by

\(T_{r + 1}\) = \((-1)^r\)\(^{n}C_r x^{n – r} a^r\)

In the binomial expansion of \((1 + x)^n\), we have

\(T_{r + 1}\) = \(^nC_r x^r\)

In the binomial expansion of \((1 – x)^n\), we have

\(T_{r + 1}\) = \((-1)^r\)\(^nC_r x^r\)

Nth term from the End :

In the binomial expansion of \((x + a)^n\), the rth term from the end is ((n + 1) – r + 1) = (n – r + 2)th term form the beginning.

Example : Write the general term in the expansion of \((x^2 – y)^6\).

Solution : We have, \((x^2 – y)^6\) = \(|(x^2 + (-y)|^6\)

The general term in the expansion of the above binomial is given by 

\(T_{r + 1}\) = \(^{n}C_r x^{n – r} a^r\)

\(\implies\) \(T_{r + 1}\) = \(^{6}C_r (x^2)^{6 – r} (-y)^r\)

\(\implies\) \(T_{r + 1}\) = \((-1)^r\)\(^{6}C_r x^{12 – 2r} y^r\)


Related Questions

Find the 9th term in the expansion of \(({x\over a} – {3a\over x^2})^{12}\).

Find the 10th term in the binomial expansion of \((2x^2 + {1\over x})^{12}\).

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