Question
Question: For 3 £ r £ n <sup>n</sup>C<sub>r</sub> + 3 <sup>n</sup>C<sub>r–1</sub>+ 3 <sup>n</sup>C<sub>r–2 +</...
For 3 £ r £ n nCr + 3 nCr–1+ 3 nCr–2 + nCr–3 is-
A
n+3Cr
B
2 n+2Cr+2
C
3 n+1Cr+1
D
3 nCr
Answer
n+3Cr
Explanation
Solution
Use nCr + nCr–1 = n+1Cr
nCr + nCr–1 + 2(nCr–1 + nCr–2) + nCr–2 + nCr–3