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Algebra Level 4

Find the number of real values x x that satisfy the equation

x + x + x = 1000. \lfloor x \rfloor + |x| + \lceil x \rceil =1000.

Notations :


Special thanks to Jason Chrysoprase for inspiring this question.
5 2 More than 6 1 0 6 3 4

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3 solutions

Hung Woei Neoh
Jul 14, 2016

Relevant wiki: Floor and Ceiling Functions - Problem Solving

x + x + x = 1000 \lfloor x \rfloor + |x| + \lceil x \rceil = 1000

For this equation, we will analyze 3 3 cases:


Case 1 : x x is a non-integer real value

Note that for non-integer real values x x , we have

Integer + + Non-integer + + Integer = = Non-integer

We will never get an integer value result. This means that no non-integer real value x x satisfies this equation.


Case 2 : x x is a non-negative integer

For non-negative integers x x , we have x = x = x = x \lfloor x \rfloor = \lceil x \rceil = |x| = x . This means we have

x + x + x = 1000 3 x = 1000 x = 1000 3 x+x+x=1000\implies 3x=1000 \implies x=\dfrac{1000}{3}

The value of x x found isn't an integer. Therefore, no non-negative integer x x satisfies this equation


Case 3 : x x is a negative integer.

For negative integers x x , we have x = x = x \lfloor x \rfloor = \lceil x \rceil = x and x = x |x|=-x . This means we have

x x + x = 1000 x = 1000 x-x+x=1000\implies x=1000

The value of x x found is non-negative. This means that no negative integer x x satisfies this equation either.


From here, we can conclude that there are 0 \boxed{0} real values x x that satisfy this equation

Nice... (+1)

Rishabh Jain - 4 years, 11 months ago

Same method, but my phone lagged and i pressed the option next to it :(

Ashish Menon - 4 years, 11 months ago

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Well, be careful next time

Hung Woei Neoh - 4 years, 11 months ago

I also know that 0 real values is the answer for my previous problem, but just to make sure, so i ask on slack, then yeah you guys know it ;)

Jason Chrysoprase - 4 years, 11 months ago

I want to ask, could x x be a complex number ?

Jason Chrysoprase - 4 years, 11 months ago

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How would we define x |x| if x x is a complex number?

Hung Woei Neoh - 4 years, 11 months ago

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|x| is well defined in complex numbers. I think you meant to say about floor and ceiling function...:-)

Rishabh Jain - 4 years, 11 months ago

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@Rishabh Jain It is? I didn't know. How do you define x |x| for complex numbers?

Hung Woei Neoh - 4 years, 11 months ago

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@Hung Woei Neoh If x = a + i b x=a+ib sich that a , b a,b are real numbers then x = a + i b = a 2 + b 2 |x|=|a+ib|=\sqrt{a^2+b^2}

Rishabh Jain - 4 years, 11 months ago

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@Rishabh Jain Ah, I see. Thanks

Hung Woei Neoh - 4 years, 11 months ago

@Hung Woei Neoh OK I thought about modulus... Obviously we cannot judge it by the definition x = x |x|=x if x is positive and x -x if x is negative..

Rishabh Jain - 4 years, 11 months ago

Good question, probably a bit overrated though. Same method, +1

Mehul Arora - 4 years, 11 months ago

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I selected Level 3. The level was then pushed to 4

Hung Woei Neoh - 4 years, 11 months ago

Wait, are x , and x \lceil x \rceil, \text{and} \lfloor x \rfloor positive for negative x?

Mehul Arora - 4 years, 11 months ago

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Nah, never. The range of x \lfloor x \rfloor is ( , 1 ) (-\infty , -1) for any negative real value of x x , while x \lceil x \rceil ranges from ( , 0 ) (-\infty , 0) for the same.

Ashish Menon - 4 years, 11 months ago

Nope. They retain the sign

Hung Woei Neoh - 4 years, 11 months ago
Josh Banister
Jul 15, 2016

Since x , x \lfloor x \rfloor, \lceil x \rceil and 1000 1000 are all intergers, 1000 x x = x 1000 - \lfloor x \rfloor - \lceil x \rceil = |x| is an integer too by closure and so is x x . Therefore x = x = x \lfloor x \rfloor = \lceil x \rceil = x . This simplifies the equation to x + 2 x = 1000 |x| + 2x = 1000 . If x x is positive then this becomes 3 x = 1000 3x = 1000 which has no integer solutions. If x x is negative then it becomes x = 1000 x = 1000 which is not a negative solution and thus rejected. If x = 0 x = 0 then 0 = 1000 0 = 1000 which is obviously false. Therefore there are no solutions.

The only answer is x = 33 1 3 x=33\frac 1 3 . The cieling and floor functions give 34 and 33. While these two and the sum are integers the absolute value function is 33 1 3 33\frac 1 3 .
So the sum is not 1000 but 1000 1 3 1000\frac 1 3 . We also know integer+integer+non integer can not add up to an integer. So no solution.

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