Back to Basics I

Find the largest natural number n n such that n 11 n-11 divides n 3 + 83 n^3 + 83 .


The answer is 1425.

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.

8 solutions

Otto Bretscher
Nov 15, 2015

Let m = n 11 m=n-11 . Now m m must divide n 3 + 83 = ( m + 11 ) 3 + 83 = m 3 + 33 m 2 + 363 m + 1414 n^3+83=(m+11)^3+83=m^3+33m^2+363m+1414 . Thus m m must divide 1414; the largest such m m is m = 1414 m=1414 . Finally, n = m + 11 = 1425 n=m+11=\boxed{1425}

Moderator note:

Simple standard approach for divisibility of a polynomial (in n), by a linear term (in n).

I did same..

Dev Sharma - 5 years, 7 months ago

Log in to reply

Me too!!!!!!!!!

Kushagra Sahni - 5 years, 7 months ago

Log in to reply

Me too... !

Manisha Garg - 5 years, 6 months ago
Praful Jain
Nov 16, 2015

This is what I did, and while it yields the correct answer, it doesn't necessarily show that the resultant is the largest natural number.

Ken Hodson - 5 years, 7 months ago

Log in to reply

No, 1425 is the maximum value because if 1425<maximum value then the remainder will not be zero.For example if the maximum value was 1426 then the divisor n-11=1415 will be greater than remainder 1414 and so it will not give 0 as the remainder for satisfying the condition of the given question.

Praful Jain - 5 years, 7 months ago
Brandon Monsen
Nov 15, 2015

We can rewrite n 3 + 83 n^{3}+83 as

n 3 + 83 = ( n 2 + a n + b ) ( n 11 ) + c n^{3}+83=(n^{2}+an+b)(n-11)+c n 3 + 83 = n 3 11 n 2 + a n 2 11 a n + b n 11 b + c n^{3}+83=n^{3}-11n^{2}+an^{2}-11an+bn-11b+c n 3 + 83 = n 3 + ( a 11 ) n 2 + ( b 11 a ) n 11 b + c n^{3}+83=n^{3}+(a-11)n^{2}+(b-11a)n-11b+c

From the above equation we can get the system:

{ a 11 = 0 b 11 a = 0 c 11 b = 83 \begin{cases} a-11=0 \\ b-11a=0 \\ c-11b=83 \\ \end{cases}

We can then solve a , b , c a,b,c one by one to get:

a = 11 b = 121 c = 1414 a=11\\ b=121\\ c=1414

Since the polynomial has a factor of n 11 n-11 in it, we only need to check for the largest value of n n such that n 11 1414 n-11|1414

This means that the greatest possible value for n n is when n 11 = 1414 n-11=1414 , since the quotient 1414 n 11 = 1 \frac{1414}{n-11}=1 . If we increase n n any further, the quotient will be less than one, and thus never again an integer.

So our greatest possible value for n n is

n 11 = 1414 n-11=1414 n = 1425 n=\boxed{1425}

You can directly show it from

n 3 1331 + 1414 n 11 \Large{\frac{n^3-1331+1414}{n-11}}

Department 8 - 5 years, 7 months ago

Log in to reply

You're right I actually never thought about it that way. The only term we care about is that "c" term which ends up just being 1 1 3 11^{3} . Nice one.

Brandon Monsen - 5 years, 7 months ago
Dominick Hing
Nov 17, 2015

According to the remainder theorem, if n 3 + 83 { n }^{ 3 }+83 is divided by n 11 n - 11 , then the remainder will be ( 11 ) 3 + 83 = 1331 + 83 = 1414 { (11)} ^ {3} + 83 = 1331 + 83 = 1414

If n 11 n - 11 divides n 3 + 83 { n }^{ 3 }+83 , then we must somehow force the remainder to become zero, since by definition, when one number divides another, the remainder is zero. The only way to do this is by setting the divided, n 11 n - 11 , equal to 1414 1414 . This works because when the dividend equals the remainder in a division problem, what really happens is the quotient increases by 1 1 and the remainder becomes zero.

For example, 63 / 7 63 / 7 can be written as 8 8 with a remainder of 7 7 , but that is really just 9 9 with no remainder.

If we set n 11 n - 11 , equal to 1414 1414 ., then n = 1425 n = 1425

Moderator note:

Great usage of the remainder factor theorem. This approach generalizes to cases where we want a linear function to divide a polynomial.

Arulx Z
Nov 15, 2015

We can use factor theorem to divide n 3 + 83 n^3+83 by n 11 n-11 and then try to maximize n n keeping the remainder 0.

11 1 0 0 83 1 1 121 1331 1 11 121 1414 \begin{matrix} 11 & | & 1 & 0 & 0 & 83 \\ & | & \underline { } & \underline { 1 } 1 & \underline { 121 } & \underline { 1331 } \\ & & 1 & 11 & 121 & 1414 \end{matrix}

(I'm sorry for bad rendering)

So now we can rewrite

n 3 + 83 n 11 = n 2 + 11 n + 121 + 1414 n 11 \frac { { n }^{ 3 }+83 }{ n-11 } ={ n }^{ 2 }+11n+121+\frac { 1414 }{ n-11 }

Since we want the remainder to be 0, we want to find an n n such that n 11 1414 n-11|1414 . So n 11 n-11 is a factor of 1414. Since we want n n to be as large as possible, let's assign the value of n 11 n-11 as 1414. Hence

n 11 = 1414 n = 1425 n-11=1414\\ n=1425

Moderator note:

Be careful with "we want n to be as large as possible, so we assign the maximum value". We must also ensure that we obtain an integer result, though that is guaranteed by the coefficient of 1.

Andrea Civitarese
Nov 19, 2015

Since

n 3 + 83 n 11 = n 2 + 11 n + 121 + 1414 n 11 \frac{n^3+83}{n-11} = n^2+11n+121+\frac{1414}{n-11}

the reminder vanishes if and only if the last fraction is a integer. The largest n for which it happens is when n 11 = 1414 n = 1425 n-11=1414 \rightarrow n = 1425

Ayush Garg
Nov 17, 2015

N 3 + 83 = ( n 3 1331 ) + 1414 N^3 + 83 = (n^3 -1331) + 1414 Since n -11 obviously divides the term in the bracket, we have to find largest value of n satisfying n 11 1414 n-11|1414 Therefore max value of n is 1425.

Kaustubh Miglani
Nov 17, 2015

its just too easy.by remainder theorem put n-11=0(to find the remainder) n=11 putting we get 11 cube +83 that is 1414 so it should be the remainder unless n is less than or equal to 1414 .now if n is a factor of 1414 then the remainder would be zero. the highest factor of number 1414 is 1414.so n-11=1414(for highest value) n=1425 too easy just some logic and basic maths follow me if u like this solution

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...