Rooted Inequalities

Algebra Level 5

How many integers satisfy ( n 23 × 24 ) 2 < 1 (\sqrt{n} -\sqrt{23\times24})^2 <1


The answer is 93.

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4 solutions

Kartik Sharma
Nov 13, 2014

( n 23 × 24 ) 2 < 1 {(\sqrt{n} - \sqrt{23 \times 24})}^{2} < 1

( n 552 ) 2 < 1 {(\sqrt{n} - \sqrt{552})}^{2} <1

( n 23.49... 2 < 1 {(\sqrt{n} - 23.49...}^{2} < 1

Now for a 2 < 1 {a}^{2} <1 , a < 1 or -1

Hence, n 22.5 o r 24.48 \sqrt{n} \approx 22.5 or 24.48 ( \approx should mean "around")

First take the case of 22.5,

hence, n 506.25 n \approx 506.25 , therefore, least value of n has to be 507

Now, taking the case of 24.48,

hence, n 599.27 n \approx 599.27 , therefore, maximum value of n has to be 599(because 600 \sqrt{600} will become more than 24.49

As a result,

all numbers from 507 to 599 can all be the values of n. So, there are in total 93 \boxed{93} numbers

@anujshikharkhane where do you get these problems?do you make them?

Adarsh Kumar - 6 years, 7 months ago

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Yes. I just like Algebra a lot.

Anuj Shikarkhane - 6 years, 7 months ago

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nice! keep it up!

Adarsh Kumar - 6 years, 6 months ago

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@Adarsh Kumar Thanks a lot!😃👍

Anuj Shikarkhane - 6 years, 6 months ago

@Adarsh Kumar It seems that you like guns a lot. Btw, I also like them.

Anuj Shikarkhane - 6 years, 6 months ago

Oh, so even you bashed it? @Kartik

Krishna Ar - 6 years, 7 months ago

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Yeah! :P You did the same, right?

Kartik Sharma - 6 years, 7 months ago

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If you know basic english, you can find the answer to your question, yourself :#

Krishna Ar - 6 years, 7 months ago

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@Krishna Ar Yeah I know but what made me ask it is that there is a possibility that you are pointing to someone else(SG etc.) and then saying that "EVEN you did by bashing", SG DID TOO.

Kartik Sharma - 6 years, 7 months ago
Sudhir Kondepati
Nov 13, 2014

(n^(1/2) - (23*24)^(1/2))^2 <1

sq.rt on both sides we get

(n^(1/2) - (23 24)^(1/2)) <1 and (n^(1/2) - (23 24)^(1/2)) >-1

n^(1/2)<1 + (23 24)^(1/2) and n^(1/2) >-1+ (23 24)^(1/2)

square on both sides

n<(1 + (23 24)^(1/2) )^2 and n >(-1+ (23 24)^(1/2) )^2

now range of n is [(1 + (23 24)^(1/2) )^2 ,(-1+ (23 24)^(1/2) )^2]

n (max) -n (min) = 4 ((23 24)^(1/2) ) =93.979 i.e no.of integers that satisfy inequalities are 93

汶良 林
Aug 18, 2015

O B
Dec 10, 2014

It's trivial to see that ( m ± 1 ) 2 = m 2 ± 2 m + 1 (m\pm1)^2=m^2\pm2m+1 . Note since ( m + 1 ) 2 m 2 = 1 \sqrt{(m+1)^2}-\sqrt{m^2}=1 it's obvious that n + k n < 1 \left|\sqrt{n+k}-\sqrt{n}\right|<1 if and only if 2 n + 1 < k < 2 n + 1 -2\sqrt{n}+1<k<2\sqrt{n}+1 .

In our case, n = 23 24 n=23\cdot24 so clearly 23 < n < 24 23<\sqrt{n}<24 and it follows that for integer k k it must be that 2 23 + 1 k 2 23 + 1 -2\cdot23+1\le k\le2\cdot23+1 giving a total of 4 23 + 1 = 93 4\cdot23+1=93 solutions.

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