NT is quite popular

Find the largest positive integer n n for which n 3 + 100 n^3+100 is divisible by n + 10 n+10 .


The answer is 890.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Nihar Mahajan
Mar 15, 2016

So we want to find largest integer n n such that n 3 + 100 n + 10 \dfrac{n^3+100}{n+10} is an integer,

n 3 + 100 n + 10 = n 3 + 1000 900 n + 10 = ( n + 10 ) ( n 2 10 n + 100 ) 900 n + 10 = n 2 10 n + 100 900 n + 10 \dfrac{n^3+100}{n+10} = \dfrac{n^3+1000-900}{n+10}=\dfrac{(n+10)(n^2-10n+100)-900}{n+10} =n^2-10n+100 -\dfrac{900}{n+10}

For the above expression to be an integer , the largest value of n n is 890 890 .

Our methods are more or less the same :-P

Akshat Sharda - 5 years, 3 months ago

Log in to reply

I did the ques in mind

Department 8 - 5 years, 3 months ago

Log in to reply

Me too!....

Nihar Mahajan - 5 years, 3 months ago

Me not!......

(<!--unnecessary comment--)

Harsh Shrivastava - 5 years, 3 months ago

Can you explain the last step of your solution please. If n=890 then 900 890 \frac{900}{890} will be in fraction and thus our whole answer will be in fraction.So n + 10 n+10 should be the greatest factor of 900 , then only are answer will be an integer(i.e greatest value of n=440) Please explain

Chirayu Bhardwaj - 5 years, 3 months ago

Log in to reply

If n = 890 n=890 , 900 n + 10 = 900 900 = 1 \dfrac{900}{n+10} = \dfrac{900}{900}=1 .

Nihar Mahajan - 5 years, 3 months ago

Log in to reply

Firstly it should be 10 not 1 and second if n n =889 then it is 889+10=899 and 900 is not divisible by 899 and thus our answer will be fraction again.

Chirayu Bhardwaj - 5 years, 3 months ago

Log in to reply

@Chirayu Bhardwaj n = 890 n=890 and not 889.

Nihar Mahajan - 5 years, 3 months ago

Log in to reply

@Nihar Mahajan It is an inequality that n 890 n\leq890 so n n could be 889.

Chirayu Bhardwaj - 5 years, 3 months ago

Log in to reply

@Chirayu Bhardwaj The question asks for largest value of n n .

Nihar Mahajan - 5 years, 3 months ago

Log in to reply

@Nihar Mahajan Okay ! thank you .+1

Chirayu Bhardwaj - 5 years, 3 months ago

Log in to reply

@Chirayu Bhardwaj No problem :)

Nihar Mahajan - 5 years, 3 months ago

@Chirayu Bhardwaj this question is not valid for n smaller than 890 . i think. correct me if i am wrong

Chirayu Bhardwaj - 5 years, 3 months ago

Log in to reply

@Chirayu Bhardwaj I confused inequality with maximum value , sorry for being silly :(

Nihar Mahajan - 5 years, 3 months ago

Same solution!!

Aakash Khandelwal - 5 years, 3 months ago

Nice solution. But instead of division, you could have used remainder theorem.

Arulx Z - 5 years, 2 months ago

I also did the same....

Aditya Kumar - 5 years, 1 month ago
Akshat Sharda
Mar 15, 2016

Let,

n 3 + 100 = ( n 2 + a n + b ) ( n + 10 ) + c = n 3 + n 2 ( 10 + a ) + n ( b + 10 a ) + 10 b + c { 10 + a = 0 a = 10 b + 10 a = 0 b = 100 c + 10 b = 100 c = 900 gcd ( n 3 + 100 , n + 10 ) = gcd ( 900 , n + 10 ) = gcd ( 900 , n + 10 ) Maximum value of n = 890 \begin{aligned} n^3+100 & =(n^2+an+b)(n+10)+c \\ & = n^3+n^2(10+a)+n(b+10a)+10b+c \\ \end{aligned} \\ \begin{cases} 10+a &= 0 \Rightarrow a=-10 \\ b+10a & = 0 \Rightarrow b=100 \\ c+10b & = 100 \Rightarrow c=-900 \end{cases} \\ \gcd(n^3+100,n+10)=\gcd(-900,n+10)=\gcd(900,n+10) \\ \text{Maximum value of }n=\boxed{890}

I believe you mean " + c " in the second step.

Aditya Sky - 5 years, 3 months ago

Log in to reply

Yeah! It was a typo. Thanks!

Akshat Sharda - 5 years, 3 months ago

Nice solution have you solved my logical question named KILLING SEQUENCE?(available on my profile)

Atanu Ghosh - 5 years, 3 months ago

Log in to reply

No, I haven't solved it.

Akshat Sharda - 5 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...