If r=113 \ 1 ( 4 + (r - 1) 6 ) ( 4 + r 6 ) \ = a 3 + b, a, b Z, then a2 + b2 is equal to :
- 10
- 4
- 8
- 2
Answer
(3) 8
Solution
Let Ar = 4 + (r - 1) 6, Br = 4 + r 6 Br - Ar = 6
1 A B = 1 6 (B - A) A B = 2( A - B)
Sum = 2 r=113( Ar - Br) (telescoping, as Br = Ar+1)
= 2 [ 4 - ( 4 + 13 6 ) ]
4 + 13 6 = 2 + 5 12
( 4 + 13 6 ) = 5 12 = 2 - 3
Sum = 2[1 - (2 - 3)] = 2 3 - 2
a = 2, b = -2
a2 + b2 = 4 + 4 = 8
Answer: (3)