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139(A)
Найти сумму:

(1)/(1+sqrt2)+(1)/(sqrt2+sqrt3)+dots+(1)/(sqrt2006+sqrt2007)

139( A) 139(\mathrm{~A}) \newlineНайти сумму:\newline11+2+12+3++12006+2007 \frac{1}{1+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+\ldots+\frac{1}{\sqrt{2006}+\sqrt{2007}}

Full solution

Q. 139( A) 139(\mathrm{~A}) \newlineНайти сумму:\newline11+2+12+3++12006+2007 \frac{1}{1+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+\ldots+\frac{1}{\sqrt{2006}+\sqrt{2007}}
  1. Multiply by Conjugate: We can use the method of conjugates to simplify each term. Multiply the numerator and denominator by the conjugate of the denominator.\newlineFor the first term: (11+2)×(1212)=1212(\frac{1}{1+\sqrt{2}}) \times (\frac{1-\sqrt{2}}{1-\sqrt{2}}) = \frac{1-\sqrt{2}}{1-2}.
  2. Simplify First Term: Simplify the first term: (12)/(1)=21(1-\sqrt{2})/(-1) = \sqrt{2}-1.
  3. Apply Method to All Terms: Apply the same method to all terms in the series. Each term will look like (n+1n)(\sqrt{n+1}-\sqrt{n}).
  4. Telescoping Effect: Notice that when we add the terms, there will be a telescoping effect. Most terms will cancel out, leaving only the first and last parts of the square roots from the first and last terms.
  5. Add Simplified Terms: Add the simplified terms: (21)+(32)++(20072006)(\sqrt{2}-1) + (\sqrt{3}-\sqrt{2}) + \ldots + (\sqrt{2007}-\sqrt{2006}).
  6. Final Cancellation: After cancellation, we're left with: 1+2007.-1 + \sqrt{2007}.
  7. Final Answer: The final answer is 20071\sqrt{2007} - 1.

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