Determine the solutions

Determine all pairs of positive integers ( x , y ) (x,y) such that the expression below is an integer. x 2 y + x + y x y 2 + y + 11 \large \frac{x^2y+x+y}{xy^2+y+11}

Note: If the solutions are of the form ( x 1 , y 1 ) , ( x 2 , y 2 ) , ( x 3 , y 3 ) , ( a k 2 , b k ) (x_1,y_1) , (x_2,y_2) , (x_3,y_3) , (ak^2,bk) , where k k is a variable integer, then find x 1 + y 1 + x 2 + y 2 + x 3 + y 3 + a + b x_1+y_1+x_2+y_2+x_3+y_3+a+b .


The answer is 164.

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1 solution

Patrick Corn
Feb 15, 2018

Multiply by y y and subtract x x to get x 2 y 2 + x y + y 2 x y 2 + y + 11 x = y 2 11 x x y 2 + y + 11 . \frac{x^2y^2+xy+y^2}{xy^2+y+11} - x = \frac{y^2-11x}{xy^2+y+11}. This must also be an integer. If y 2 11 x y^2-11x is positive, this is impossible, because the numerator will be less than the denominator. If y 2 = 11 x , y^2=11x, then y = 11 k y=11k for some integer k k and x = 11 k 2 . x=11k^2. The tuple ( 11 k 2 , 11 k ) (11k^2,11k) gives a solution for any positive integer k . k.

Finally, if y 2 < 11 x , y^2 < 11x, then we need 11 x y 2 11x-y^2 to be at least as large as the denominator: 11 x y 2 x y 2 + y + 11 x ( y 2 11 ) + ( y 2 + y + 11 ) 0. 11x-y^2 \ge xy^2 + y + 11 \Rightarrow x(y^2-11)+(y^2+y+11) \le 0. Note that if y 4 , y \ge 4, the right side equals m + n x m+nx where m m and n n are positive, which would be a contradiction.

So this leaves the cases y = 1 , 2 , 3. y=1,2,3.

If y = 1 y=1 we get x 2 + x + 1 x + 12 \frac{x^2+x+1}{x+12} is an integer, but this equals x 11 + 133 x + 12 , x-11 + \frac{133}{x+12}, and the only divisors of 133 133 larger than 12 12 are 19 19 and 133 , 133, so the two solutions are x = 7 , 121. x=7,121.

If y = 2 y=2 we get 2 x 2 + x + 2 4 x + 13 \frac{2x^2+x+2}{4x+13} is an integer, but this equals 1 2 ( x 11 x 4 4 x + 13 ) . \frac12 \left( x - \frac{11x-4}{4x+13}\right). So 11 x 4 4 x + 13 \frac{11x-4}{4x+13} is an integer. Clearly this integer is less than 3 , 3, so it's 1 1 or 2. 2. If it's 1 1 then 7 x = 17 , 7x=17, which is no good. If it's 2 2 then x = 10 , x=10, and that does in fact work (the expression is 212 53 = 4 \frac{212}{53}=4 ), so that's the one solution in this case.

If y = 3 y=3 then we get 3 x 2 + x + 3 9 x + 14 \frac{3x^2+x+3}{9x+14} is an integer, but this equals 1 3 ( x 11 x 9 9 x + 14 ) . \frac13 \left( x - \frac{11x-9}{9x+14} \right). So 11 x 9 9 x + 14 \frac{11x-9}{9x+14} is an integer. Clearly this integer is less than 2 , 2, so it's 1. 1. But this leads to 2 x = 23 , 2x=23, which is no good.

Putting it all together, our solutions are ( 7 , 1 ) , ( 121 , 1 ) , ( 10 , 2 ) , (7,1),(121,1),(10,2), and ( 11 k 2 , 11 k ) (11k^2,11k) for any k . k. The sum is 7 + 1 + 121 + 1 + 10 + 2 + 11 + 11 = 164 . 7+1+121+1+10+2+11+11=\fbox{164}.

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