Nested floors and ceilings

Algebra Level 2

6 + 6 + 6 + 6 + = ? \sqrt{ \left \lceil \lfloor \sqrt{6} \rfloor \right \rceil +\sqrt{ \left \lceil \lfloor \sqrt{6} \rfloor \right \rceil + \sqrt{ \left \lceil \lfloor \sqrt{6} \rfloor \right \rceil + \sqrt{ \left \lceil \lfloor \sqrt{6} \rfloor \right \rceil + \cdots}}}} = \, ? \

Notations :


The answer is 2.

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3 solutions

6 = 2 \lceil \lfloor \sqrt6 \rfloor \rceil=2 .

The expression is equivalent to 2 + 2 + 2 \sqrt{2 +\sqrt{2+\sqrt{2 \ldots}}}

Let x = 2 + 2 + 2 x=\sqrt{2 +\sqrt{2+\sqrt{2 \ldots}}} .

We have 2 + x = x \sqrt{2+x}=x or x 2 x 2 x^2-x-2 .

Solving the quadratic equation and taking only the positive root of x x (as the radical symbol of a number is always positive) we get x = 2 x=\boxed{2} .

Hung Woei Neoh
May 12, 2016

6 = 2.449 = 2 = 2 \left \lceil \left \lfloor \sqrt{6} \right \rfloor \right \rceil \\ =\left \lceil \left \lfloor 2.449\ldots \right \rfloor \right \rceil \\ =\left \lceil 2 \right \rceil \\ =2

Substituting it into the nested radical, we get:

2 + 2 + 2 + 2 + \sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+\ldots}}}}

Now, let the value of this expression be x x .

2 + 2 + 2 + 2 + = x 2 + 2 + 2 + 2 + = x 2 2 + x = x 2 x 2 x 2 = 0 ( x 2 ) ( x + 1 ) = 0 x = 2 , 1 \sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+\ldots}}}}=x \\ 2+\sqrt{2+\sqrt{2+\sqrt{2+\ldots}}} = x^2\\ 2+x=x^2\\ x^2-x-2=0\\ (x-2)(x+1) = 0\\ x=2,\;-1

Since we know that the square root of any number cannot result in negative numbers, we take the positive solution.

The value of the expression is 2 \boxed{2}

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