Broke the right Glass!

Find the least number lying in -100 to 100 exclusive to be subtracted from 200 to make it a perfect cube.

Details and Assumptions :

  • A number n n is called a perfect cube if we can write it as a 3 a^3 for some integer a a .

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136 16 No such number exists. None of the given. 75

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3 solutions

Sandeep Bhardwaj
May 25, 2015

Let λ \lambda be the least no. to be subtracted from 200 \color{#D61F06}{200} to make it a perfect cube.

\therefore

200 λ = a 3 200-\lambda=a^3

Perfect cube numbers nearest neighbors to 200 \color{#D61F06}{200} are 216 , 125 216, 125 .

case 1 : \Large \text{case 1 :}

Let a 3 a^3 be 125 125 for which value of a a will be 5 5 .

200 λ = 125 \implies 200-\lambda=125

λ = 75 \implies \lambda=75

case 2 : \Large \text{case 2 :}

Let a 3 a^3 be 216 216 for which value of a a will be 6 6 .

200 λ = 216 \implies 200-\lambda=216

λ = 16 \implies \lambda=-16

\color{#20A900}{\bullet} If you take any other case than the above two, the value goes out of the range ( 100 , 100 ) (-100,100)

Out of the above two values of λ \lambda the least value is 16 \boxed{-16} . (which is not given in the options)

So, the correct option is None of the given.

enjoy !

Moderator note:

This solution actually hides the "proper" way to approach the question. Instead of suggesting that "perfect cube nearest to 200 are 216, 125", it should instead be phrased as "We are interested in the region 100 , 300 100, 300 , and the only perfect cubes here are 125 and 216. Thus, the smallest is 200 216 = 16 200-216 = -16 ".

While Sandeep did essentially say all of that eventually, the phrasing and ordering of the solution made the crux of it much more obscure.

Let me see how many of Brilliant guys are trapped in my trick. :P

Upvote this comment if you got trapped.

Thanks !

Sandeep Bhardwaj - 6 years ago

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But I am not trapped.

Rama Devi - 6 years ago

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then you can upvote the solution. :P

Sandeep Bhardwaj - 6 years ago

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@Sandeep Bhardwaj I will surely do it

Rama Devi - 6 years ago

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@Rama Devi Thank you ¨ \ddot \smile for you kind Job. :P

Sandeep Bhardwaj - 6 years ago

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@Sandeep Bhardwaj Then why don't you try my new set - Logic ?

Rama Devi - 6 years ago

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@Rama Devi I'll try surely.

Sandeep Bhardwaj - 6 years ago

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@Sandeep Bhardwaj Try to be in the top of the leader board.

Rama Devi - 6 years ago

Oh come on!!!Guess I'm the first to be caught in your net eh?Nice trap btw sir Sandeep!

Arian Tashakkor - 6 years ago

Sandeep sir i am preparing for RMO and also an aspirant for IIT-JEE. So, will you please suggest me some good books for especially maths and physics.......

Abhisek Mohanty - 6 years ago

Objection! λ = 143 \lambda=-143 gives 200 λ = 7 3 200-\lambda=7^3 , and 143 < 16 -143 < -16 . And so on for 8 3 , 9 3 8^3, 9^3 ... Hence no such number exists.

Tijmen Veltman - 6 years ago

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Ohhh!! Thank you for pointing it out. I forgot to use that condition. Now I mentioned that. Sorry for the inconvenience caused !

Sandeep Bhardwaj - 6 years ago

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Great! Remember, don't stop breaking after the first glass ;)

Tijmen Veltman - 6 years ago
Rama Devi
Jun 3, 2015

The answer is -16 , which when subtracted from 200 , gives the answer as 216 , which is the nearest perfect cube to 200.

Moderator note:

Why? Where's your working?

Chew-Seong Cheong
May 26, 2015

Let n n be the integer 100 < n < 100 -100 < n < 100 and m = 200 n m = 200-n .

  • The smallest m m , m m i n = 200 100 = 100 m_{min} = 200-100 = 100 and the smallest perfect cubic p m i n > 100 p_{min} > 100 , therefore p m i n = 100 3 3 = 5 3 = 125 p_{min} = \left \lceil \sqrt[3]{100} \right \rceil ^3 = 5^3 = 125 . 200 n 1 = 125 200-n_1 = 125 n 1 = 75 \Longrightarrow n_1 = 75 .
  • The greatest m m , m m a x = 200 ( 100 ) = 300 m_{max} = 200-(-100) = 300 and the largest perfect cubic p m a x < 300 p_{max} < 300 , therefore p m a x = 300 3 3 = 6 3 = 216 p_{max} = \left \lfloor \sqrt[3]{300} \right \rfloor ^3 = 6^3 = 216 . 200 n 2 = 216 200-n_2 = 216 n 2 = 16 \Longrightarrow n_2 = -16 .

Therefore, the smallest n m i n = n 2 = 16 n_{min} = n_2 = \boxed{-16}

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