1 + 5 + 9 + ............ + 81 = কত?
Solution
Correct Answer: Option B
১ম পদ a =1
সাধারন অন্তর d = (5 - 1) = 4
n তম পদ = a + (n - 1)d
⇒ 81 = 1 + (n - 1) 4
⇒ 81 = 1 + 4n - 4
⇒ 4n = 84
⇒ n = 21
সমষ্টি Sn = (n/2) {2a + (n - 1)d}
= (21/2) {2 × 1 + (21 -1)4}
= (21/2) × 82
= 861
সাধারন অন্তর d = (5 - 1) = 4
n তম পদ = a + (n - 1)d
⇒ 81 = 1 + (n - 1) 4
⇒ 81 = 1 + 4n - 4
⇒ 4n = 84
⇒ n = 21
সমষ্টি Sn = (n/2) {2a + (n - 1)d}
= (21/2) {2 × 1 + (21 -1)4}
= (21/2) × 82
= 861
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