If r=010 ( 10r+1 - 110r ) 11Cr+1 = 11 - 11111010, then is equal to :
- 11
- 20
- 24
- 15
Answer
(2) 20
Solution
10r+1 - 110r = 10 - 110r
LHS = 10 r=010 11Cr+1 - r=010 11Cr+110r
Put k = r + 1 (k = 1 to 11):
k=111 11Ck = 211 - 1
k=111 11Ck10k-1 = 10 k=111 11Ck ( 110 )k = 10 [ ( 1110 )11 - 1 ]
LHS = 10(211 - 1) - 10 ( 11111011 - 1 )
= 10 211 - 11111010
= 211 1011 - 11111010 = 2011 - 11111010
= 20
Answer: (2)