Infinity radicals

Algebra Level 5

the value of 5 + 15 2 + 3 2 + 3 8 + 3 128 + 3 12 8 2 × 2 . . . \sqrt{5+\sqrt{\dfrac{15}{2}+\sqrt{\dfrac{3}{2}+\sqrt{\dfrac{3}{8}+\sqrt{\dfrac{3}{128}+\sqrt{\dfrac{3}{128^2 \times 2}...\infty}}}}}} can be expressed as n \sqrt{n} . find n this is part of the set the Radicals


The answer is 8.

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1 solution

Aareyan Manzoor
Dec 15, 2014

3 2 + 3 8 + 3 128 + 3 12 8 2 × 2 . . . \sqrt{\dfrac{3}{2}+\sqrt{\dfrac{3}{8}+\sqrt{\dfrac{3}{128}+\sqrt{\dfrac{3}{128^2 \times 2}...\infty}}}} can be written as 3 2 + 1 2 3 2 + 1 2 3 2 + 1 2 . . . \sqrt{\dfrac{3}{2}+\dfrac{1}{2}\sqrt{\dfrac{3}{2}+\dfrac{1}{2}\sqrt{\dfrac{3}{2}+\dfrac{1}{2}...\infty}}} which is 3 2 + 1 2 x = x \sqrt{\dfrac{3}{2}+\dfrac{1}{2}x}=x solve x = 1.5 x=1.5 now insert tis value and solve

I practically solved it by estimation... 8 is small enough to estimate ( and the answer most probably wouldnt be 9)

Aloysius Ng - 6 years, 5 months ago

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I too did the same way.

Niranjan Khanderia - 2 years, 10 months ago

Nice problem! I liked how the quadratic solved nicely.

Michael Ng - 6 years, 5 months ago

Aareyan, I am sorry that you had to deal with a message from a troll. I have since deleted the conversation thread, and also deactivated the account.

That behavior is not tolerated on Brilliant.

Calvin Lin Staff - 6 years, 5 months ago

How can I figure out how a particular statement can also be written as? I solved the problem using estimation but I would also like to know the method in which problems like these are solved.

Priontu Chowdhury - 5 years, 11 months ago

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