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Question
1 + 5 + 9+ ------81 = ?
21
861
840
860
ANSWER : 2
Descrption
<pre> 1 + 5 + 9 + ....... + 81 ধারাটির , n - তম পদ = a + (n - a)d বা, 81 = 1 + (n - 1) 4 বা, 81 = 1 + n - 4 বা 4n = 84 ∴ n = 21 ∴ সমষ্টি = <math xmlns = "http://www.w3.org/1998/Math/MathML"><mfrac><mn>21</mn><mn>2</mn></mfrac><mo>{</mo><mn>2</mn><mo>.</mo><mn>1</mn><mo> + </mo><mo>(</mo><mn>21</mn><mo> - </mo><mn>1</mn><mo>)</mo><mn>4</mn><mo>}</mo><mspace linebreak = "newline"/><mo> = </mo><mfrac><mn>21</mn><mn>2</mn></mfrac><mo>(</mo><mn>2</mn><mo> + </mo><mn>80</mn><mo>)</mo><mspace linebreak = "newline"/><mo> = </mo><mn>21</mn><mo>×</mo><mn>41</mn><mspace linebreak = "newline"/><mo> = </mo><mn>861</mn></math> </pre>
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