Question

Find maximum sum of given series

94 + 89 + 84 + 79 + ....... =

• A. 931
• B. 921
• C. 941
• D. None of these

Answer 1

Answer: (A)

931

Answer 2

Let t_n = first –ve term

94 + (n –1) (–5) < 0 ̃ 94 – 5n + 5 < 0

99 < 5n ̃ n > 19.8

nÎN ̃ n = 20, 21, 22…..

̃ t₂₀, t₂₁, t₂₂…… are all –ve terms

S_max = S₁₉ = $\frac { 19 } { 2 } [ 2 \times 94 + ( 19 - 1 ) ( - 5 ) ]$

= $\frac { 19 } { 2 } \times [ 188 - 90 ]$ = 19 × 49 = 931