nC0 + nC3 + nC6 + nC9 + ...


what does this equal to?

Verified Answer :

The value of \(^nC_0 + ^nC_3 + ^nC_6 + ^nC_9\) is \(1 + \frac{n!}{6}[\frac{1}{(n - 3)!}+\frac{1}{120(n - 6)!} + \frac{1}{60480(n - 9)!}]\)

The expression is given as:

\(^nC_0 + ^nC_3 + ^nC_6 + ^nC_9\)

The combination formula is:

\(^nC_r = \frac{n!}{(n - r)!r!}\)

So, we have:

\(\frac{n!}{(n - 0)!0!} + \frac{n!}{(n - 3)!3!}+\frac{n!}{(n - 6)!6!} + \frac{n!}{(n - 9)!9!}\)

Evaluate the factorials

\(\frac{n!}{(n - 0)!} + \frac{n!}{6(n - 3)!}+\frac{n!}{720(n - 6)!} + \frac{n!}{362880(n - 9)!}\)

Evaluate the difference

\(\frac{n!}{n!} + \frac{n!}{6(n - 3)!}+\frac{n!}{720(n - 6)!} + \frac{n!}{362880(n - 9)!}\)

Evaluate the quotient

\(1 + \frac{n!}{6(n - 3)!}+\frac{n!}{720(n - 6)!} + \frac{n!}{362880(n - 9)!}\)

Factor out n!

\(1 + n![\frac{1}{6(n - 3)!}+\frac{1}{720(n - 6)!} + \frac{1}{362880(n - 9)!}]\)

Factor out 1/6

\(1 + \frac{n!}{6}[\frac{1}{(n - 3)!}+\frac{1}{120(n - 6)!} + \frac{1}{60480(n - 9)!}]\)

Hence, the value of \(^nC_0 + ^nC_3 + ^nC_6 + ^nC_9\) is

\(1 + \frac{n!}{6}[\frac{1}{(n - 3)!}+\frac{1}{120(n - 6)!} + \frac{1}{60480(n - 9)!}]\)

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