Question:

The coefficient of $x^7$ in the expression $\left(1+x\right)^{10}+x\left(1+x\right)^{9}+x^{2}\left(1+x\right)^{8}+...+x^{10}$ is :

Updated On: Feb 14, 2025
  • $420$
  • $330$
  • $210$
  • $120$
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The Correct Option is B

Solution and Explanation

Coefficient of $x^7$ is $^{10}C_{7}+^{9}C_{6}+^{8}C_{5}+...+^{4}C_{1}+^{3}C_{0}$ $\underbrace{^4C_0+^4C_1}_{^5C_1}$$+^{5}C_{2}+...+^{10}C_{7}=^{11}C_{7}=330$
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Concepts Used:

Binomial Expansion Formula

The binomial expansion formula involves binomial coefficients which are of the form 

(n/k)(or) nCk and it is calculated using the formula, nCk =n! / [(n - k)! k!]. The binomial expansion formula is also known as the binomial theorem. Here are the binomial expansion formulas.

This binomial expansion formula gives the expansion of (x + y)n where 'n' is a natural number. The expansion of (x + y)n has (n + 1) terms. This formula says:

We have (x + y)n = nC0 xn + nC1 xn-1 . y + nC2 xn-2 . y2 + … + nCn yn

General Term = Tr+1 = nCr xn-r . yr

  • General Term in (1 + x)n is nCr xr
  • In the binomial expansion of (x + y)n , the rth term from end is (n – r + 2)th .