Aspire Faculty ID #3627 · Topic: NIMCET 2012 · Just now
NIMCET 2012

The sum $^{20}C_8 + ^{20}C_9 + ^{21}C_{10} + ^{22}C_{11} - ^{23}C_{11}$

Solution

Use Pascal identity: $^{n}C_r + ^{n}C_{r+1} = ^{n+1}C_{r+1}$ 
So, $^{20}C_8 + ^{20}C_9 = ^{21}C_9$ 
Then, $^{21}C_9 + ^{21}C_{10} = ^{22}C_{10}$
Next, $^{22}C_{10} + ^{22}C_{11} = ^{23}C_{11}$ 
Thus the expression becomes: 
$^{23}C_{11} - ^{23}C_{11} = 0$ 
$\therefore$ the answer is 0.

Previous 10 Questions — NIMCET 2012

Nearest first

Next 10 Questions — NIMCET 2012

Ascending by ID
Ask Your Question or Put Your Review.

loading...