If 26 ( 233 (12C2 )+ 255 (12C4 )+ 277 (12C6 ) . .+ + 21313 (12C12 ) )=313- , then is equal to :
- 45
- 48
- 51
- 54
Answer
(3) 51
Solution
The general term is 2k + 1k + 1 (12Ck ), k = 2, 4, , 12.
1k + 1 12Ck = 113 13Ck + 1,
26 2k + 1k + 1 12Ck = 2 13Ck + 1 2k + 1
Putting r = k + 1 = 3, 5, , 13, LHS = 2 r = 3, 5, , 1313Cr 2r.
(1 + 2)13 - (1 - 2)13 = 2 r odd13Cr 2r, we get r odd13Cr 2r = 313 + 12.
The term r = 1 is 13C1 2 = 26, so
LHS = 2 ( 313 + 12 - 26 ) = 313 + 1 - 52 = 313 - 51
∴ = 51
Answer: (3)