n + 1 0 n 3 + 1 0 0
Find the largest positive integer n such that the above expression is an integer.
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Write a comment or ask a question... Where does x even exist in this equation?
It's a dummy variable, a placeholder. Change of variables is a useful algebraic problem-solving technique.
We can write, n + 1 0 ∣ n 3 + 1 0 0
⇒ n + 1 0 ∣ n 2 ( n + 1 0 ) − 1 0 n ( n + 1 0 ) + 1 0 0 ( n + 1 0 ) − 9 0 0
⇒ n + 1 0 ∣ 9 0 0
⇒ ( n + 1 0 ) m a x = 9 0 0
⇒ n m a x = 8 9 0
(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.
n + 1 0 n 3 + 1 0 0 = n + 1 0 n 3 + 1 0 0 0 − 9 0 0 = n 2 − 1 0 n + 1 0 0 − n + 1 0 9 0 0
For n + 1 0 n 3 + 1 0 0 to be an integer, n + 1 0 9 0 0 must also be an integer.
The largest n possible occurs when n + 1 0 9 0 0 = 1 , which means n + 1 0 = 9 0 0
Hence, n = 8 9 0
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
Let n + 1 0 n 3 + 1 0 0 = a + n + 1 0 b N o w u s i n g p a r t i a l f r a c t i o n s : a ( n + 1 0 ) + b = n 3 + 1 0 0 ⇒ l e t n = − 1 0 ⇒ b = − 9 0 0 r e a r r a n g i n g w h a t ′ s l e f t o v e r : a = n + 1 0 n 3 + 1 0 0 0 = n + 1 0 ( n + 1 0 ) 3 − 3 0 n ( n + 1 0 ) = ( n + 1 0 ) 2 − 3 0 n ⇒ n + 1 0 n 3 + 1 0 0 = ( n + 1 0 ) 2 − 3 0 n − n + 1 0 9 0 0 ∴ m a x ( n ) = 8 9 0
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To make it easier put x = n + 1 0 ⟹ n = x − 1 0
So n + 1 0 n 3 + 1 0 0 reduces to x ( x − 1 0 ) 3 + 1 0 0 = x ( x 3 − 3 0 x 2 + 3 0 0 x − 1 0 0 0 ) + 1 0 0
= x 2 − 3 0 x + 3 0 0 − x 9 0 0
The largest divisor of 900 is 900 itself,
Therefore n = 9 0 0 − 1 0 = 8 9 0