1.

What will be the sum to n terms of the series 7 + 77 + 777 +...? (a) 7(10n − 9n)/81 (b) 7(10n + 1 −10 − 9n)/81 (c) 7(10n − 1 − 10) (d) 7(10n + 1 − 10) 

Answer»

Correct option (b) 7(10n + 1 −10 − 9n)/81

Explanation:

Solve this one through trial and error. For n = 2 terms the sum upto 2 terms is equal to 84. Putting n in the options it can be seen that for option (b) the sum to two terms would be given by 7 × (1000 − 10 − 18)/81 = 7 × 972/81 = 7 × 12 = 84.



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