श्रृंखला \(\rm \sqrt{5}+ \sqrt{20}+\sqrt{45}+\sqrt{80}+\ ...\) के पहले 20 पदों का योग क्या है?

  1. 300√5
  2. 200√5
  3. 210√5
  4. 420√5

Answer (Detailed Solution Below)

Option 3 : 210√5
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Detailed Solution

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संकल्पना:

1 से n तक के क्रमागत संख्याओं का योग:

\( \rm 1 + 2 + 3 +\ ...\ + n = \dfrac{n(n+1)}{2}\)

गणना:

दी गयी श्रृंखला में पहले 20 पदों के योग को निम्न रूप में लिखा जा सकता है:

\(\rm \sqrt{5}+ \sqrt{20}+\sqrt{45}+\sqrt{80}+\ ...\)

\(\rm =\sqrt{5}+ 2\sqrt{5}+3\sqrt{5}+4\sqrt{5}+\ ...\ +20\sqrt5\)

\(\rm =\sqrt5(1+2+\ ...\ +20)\)

\(\rm =\sqrt5\times \dfrac{20\times21}{2}=210\sqrt5\)

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