1
The sum of the infinite series -1 ( 74 )+ -1 ( 194 )+ -1 ( 394 )+ -1 ( 674 )+ is :
- 2+ -1 ( 12 )
- 2- -1 ( 12 )
- 2- -1 ( 12 )
- 2+ -1 ( 12 )
Answer
(3) 2- -1 ( 12 )
Solution
Numerators 7, 19, 39, 67, have second differences constant (12, 20, 28): they are 4n2+3
Tn= -1 ( 4n2+34 )= -1 (n2+ 34 )= -1 11+ (n2- 14 )
Tn= -1 (n+ 12 )- (n- 12 )1+ (n+ 12 ) (n- 12 )
Tn= -1 (n+ 12 )- -1 (n- 12 )
Sn= r=1nTr= -1 (n+ 12 )- -1 ( 12 ) (telescoping)
As n , -1 (n+ 12 ) 2
S = 2- -1 ( 12 )
Answer: (3)
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