Question:

The value of $(^{21}C_{1} - ^{10}C_{1}) + (^{21}C_{2} - ^{10}C_{2}) + (^{21}C_{3} - ^{10}C_{3}) +(^{21}C_{4} - ^{10}C_{4}) +....+(^{21}C_{10} - ^{10}C_{10})$ is :

Updated On: June 02, 2025
  • $2^{21} - 2^{10}$
  • $2^{20} - 2^{9}$
  • $2^{20} - 2^{10}$
  • $2^{21} - 2^{11}$
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The Correct Option is C

Solution and Explanation

$= ^{21}C_{1}+^{21}C_{2}+...+^{21}C_{10} \frac{1}{2}\left\{^{21}C_{0}+^{21}C_{1}+...+^{21}C_{21}\right\}-1$ $= 2^{20}-1$ $\left(^{10}C_{1}+^{10}C_{2}+...+^{10}C_{10}\right) = 2^{10}-1$ $\therefore$ Required sum $= \left(2^{20} - 1\right) - \left(2^{10} - 1\right)$ $= 2^{20} - 2^{10}$
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JEE Main Notification

Concepts Used:

Binomial Theorem

The binomial theorem formula is used in the expansion of any power of a binomial in the form of a series. The binomial theorem formula is 

Properties of Binomial Theorem

  • The number of coefficients in the binomial expansion of (x + y)n is equal to (n + 1).
  • There are (n+1) terms in the expansion of (x+y)n.
  • The first and the last terms are xn and yn respectively.
  • From the beginning of the expansion, the powers of x, decrease from n up to 0, and the powers of a, increase from 0 up to n.
  • The binomial coefficients in the expansion are arranged in an array, which is called Pascal's triangle. This pattern developed is summed up by the binomial theorem formula.