Floor Ceiling Ceiling Floor: JEE Style

Algebra Level 5

x x + x x = 1729 \large \lceil x \lfloor x \rfloor \rceil + \lfloor x \lceil x \rceil \rfloor = 1729

If the range of positive x x satisfy the equation above is α x β \alpha \leq x \leq \beta .

What is the value of 30 α + 29 β + 1 30\alpha + 29 \beta+1 if we know that it is an integer?

Inspired by: Floor Ceiling Ceiling Floor .


The answer is 1730.

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1 solution

This is the shortest way to solve the problem:

Plugging in integer values for x x quickly tells us that 29 < x < 30 29<x<30 .

Assuming x x is not an integer: x = x + 1 \left\lceil x \right\rceil =\left\lfloor x \right\rfloor +1

29 x + 30 x = 1728 \Rightarrow \left\lfloor 29x \right\rfloor +\left\lfloor 30x \right\rfloor =1728

Now, using inequalities and properties of floor function, we have:

1728 59 α < 1730 1728\le 59\alpha <1730

1728 < 59 β < 1730 1728<59\beta <1730

1728 < 30 α + 29 β < 1730 \Rightarrow 1728<30\alpha +29\beta <1730

Since the answer is an integer, 30 α + 29 β 30\alpha +29\beta is also an integer. Therefore, 30 α + 29 β + 1 = 1729 + 1 = 1730 30\alpha +29\beta+1=1729+1=1730 .

For a more straightforward and rigorous approach, look at the solution to PI Han Goh's original problem .

Moderator note:

Pi Han Goh brings up a valid point. What would the approach be if we are not given that the expression is an integer?

@Pi Han Goh , I had initially intended to show that your problem could be solved this way. But I accidentally typed in the wrong answer, sorry for the inconvenience. The fact that the answer is an integer gives it away.

Raghav Vaidyanathan - 6 years, 1 month ago

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If we don't know that the answer must be an integer, how would you determine that 30 α + 29 β + 1 30\alpha + 29\beta + 1 equals to 1730 1730 exactly?

Pi Han Goh - 6 years, 1 month ago

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I know how to solve it otherwise. This was just to show one can solve it without actually finding the values of α , β \alpha,\beta . Often in JEE, resorting to such tricks can help solve problems easily. I actually wanted to post a solution on your problem, but there were many already, so I decided to put up a problem instead.

Raghav Vaidyanathan - 6 years, 1 month ago

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@Raghav Vaidyanathan Do you want to post a note? Or let me post a note?

Pi Han Goh - 6 years, 1 month ago

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@Pi Han Goh It was your problem, hence it is only right that you do it.

Raghav Vaidyanathan - 6 years, 1 month ago

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Maybe we could prove the hypothesis that the answer will always be equal to the RHS of the floorceilingceilingfloor equation.

Raghav Vaidyanathan - 6 years, 1 month ago

@Pi Han Goh Sir , does the equality hold at the upper limit of the range. According to me it should be an open interval on the upper limit.

Ankit Kumar Jain - 2 years, 10 months ago

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@Ankit Kumar Jain @Pi Han Goh Please do reply (though I know that it has been three years since this problem was posted , it is a long time ..still.)

Ankit Kumar Jain - 2 years, 10 months ago

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@Ankit Kumar Jain I don't understand your question at all. The claim clearly states that we have to first find α \alpha and β \beta .

Pi Han Goh - 2 years, 10 months ago

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@Pi Han Goh What I mean is that the value of beta doesn't satisfy the equation .

Ankit Kumar Jain - 2 years, 10 months ago

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@Ankit Kumar Jain That makes no sense. I have to find the range of x x first. If β \beta^* is not a root of x x , then by definition β = β \beta = \beta^* cannot be true.

Pi Han Goh - 2 years, 10 months ago

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