1+3+5+ ……. +21 = কত?
Solution
Correct Answer: Option D
ধারাটির ১ম পদ a = 1,
সাধারণ অন্তর d = 3 - 1 = 2
সাধারণ অন্তর d = 3 - 1 = 2
n তম পদ = a + (n - 1)d
=> 21 = 1 + (n - 1) * 2
=> 21 = 1 + 2n - 2
=> 21 = 1 + 2n - 2
=> 2n = 22
n = 11
n = 11
1 + 3 + 5 + ...... + 31 = (11)2 = 121
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