Once you get hold of it, it's easy -Part 2

n 3 + 100 n + 10 \displaystyle \dfrac{n^{3}+100}{n+10}

Find the largest positive integer n n such that the above expression is an integer.

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The answer is 890.

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6 solutions

Discussions for this problem are now closed

Anandhu Raj
Feb 10, 2015

To make it easier put x = n + 10 n = x 10 x=n+10\Longrightarrow n=x-10

So n 3 + 100 n + 10 \displaystyle \dfrac{n^{3}+100}{n+10} reduces to ( x 10 ) 3 + 100 x = ( x 3 30 x 2 + 300 x 1000 ) + 100 x \frac { { (x-10) }^{ 3 }+100 }{ x } =\frac { { (x }^{ 3 }-30{ x }^{ 2 }+300x-1000)+100 }{ x }

= x 2 30 x + 300 900 x =x^{ 2 }-30x+300-\frac { 900 }{ x }

The largest divisor of 900 is 900 itself,

Therefore n = 900 10 = 890 n=900-10=\boxed{890}

Write a comment or ask a question... Where does x even exist in this equation?

Christian Brouillette - 6 years, 4 months ago

It's a dummy variable, a placeholder. Change of variables is a useful algebraic problem-solving technique.

Michael Lee - 6 years, 4 months ago

We can write, n + 10 n 3 + 100 n+10 | n^3+100

n + 10 n 2 ( n + 10 ) 10 n ( n + 10 ) + 100 ( n + 10 ) 900 \Rightarrow n+10| n^2(n+10)-10n(n+10)+100(n+10)-900

n + 10 900 \Rightarrow n+10|900

( n + 10 ) m a x = 900 \Rightarrow (n+10)_{max}=900

n m a x = 890 \Rightarrow n_{max}=890

Gamal Sultan
Feb 10, 2015

(n³ + 100)/(n+10) = (n³ + 1000- 900)/(n+10) = n² -10 n+100 - 900/(n+10)

Then

n +10 is a divisor of 900

The greatest divisor of 900 is 900

Then

to obtain the greatest value of n, we must have n + 10 = 900

So

n = 890

I did the same thing as you did.

Nguyen Tra - 6 years, 4 months ago

n 3 + 100 n + 10 = n 3 + 1000 900 n + 10 = n 2 10 n + 100 900 n + 10 \frac {n^3 + 100} {n+10} =\frac {n^3 + 1000 - 900} {n+10} =n^2 - 10n + 100 - \frac {900} {n+10}

For n 3 + 100 n + 10 \frac {n^3 + 100} {n+10} to be an integer, 900 n + 10 \frac {900} {n+10} must also be an integer.

The largest n n possible occurs when 900 n + 10 = 1 \frac {900} {n+10} =1 , which means n + 10 = 900 n + 10 = 900

Hence, n = 890 n = \boxed {890}

Rodrigo Escorcio
Feb 10, 2015

n³ + 100 = n³ + 10³ - 900 = (n+10)(n²-10n+100) - 900

So 900 must be divisible by n+10.

The largest divisor of 900 is 900, so n = 890

Curtis Clement
Feb 27, 2015

Let n 3 + 100 n + 10 = a + b n + 10 \frac{n^3 + 100}{n+10} = a + \frac{b}{n+10} N o w u s i n g p a r t i a l f r a c t i o n s : \large \ Now \ using \ partial \ fractions: a ( n + 10 ) + b = n 3 + 100 l e t n = 10 b = 900 a(n+10) + b = n^3 +100 \Rightarrow\ let \ n = -10 \Rightarrow\ b = -900 r e a r r a n g i n g w h a t s l e f t o v e r : \large \ rearranging \ what's \ left \ over: a = n 3 + 1000 n + 10 = ( n + 10 ) 3 30 n ( n + 10 ) n + 10 = ( n + 10 ) 2 30 n a = \frac{n^3 +1000}{n+10} = \frac{(n+10)^3 - 30n(n+10)}{n+10} = (n+10)^2 -30n n 3 + 100 n + 10 = ( n + 10 ) 2 30 n 900 n + 10 m a x ( n ) = 890 \Rightarrow\frac{n^3 +100}{n+10} = (n+10)^2 -30n - \frac{900}{n+10} \therefore\ max(n) = \boxed{890}

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