If the sum of the first 20 terms of the series 4 14 + 3 12 + 14 + 4 24 + 3 22 + 24 + 4 34 + 3 32 + 34 + 4 44 + 3 42 + 44 + is mn, where m and n are coprime, then m + n is equal to :
- 423
- 421
- 422
- 420
Answer
(2) 421
Solution
Tr = 4rr4 + 3r2 + 4
r4 + 3r2 + 4 = (r2 + 2)2 - r2 = (r2 - r + 2)(r2 + r + 2)
4r = 2 [(r2 + r + 2) - (r2 - r + 2) ]
Tr = 2 [ 1r2 - r + 2 - 1r2 + r + 2 ]
(r + 1)2 - (r + 1) + 2 = r2 + r + 2, the terms cancel in pairs (telescoping):
S20 = 2 [ 11 - 1 + 2 - 1400 + 20 + 2 ] = 2 [ 12 - 1422 ]
= 1 - 1211 = 210211
m = 210, n = 211 m + n = 421
Answer: (2)