How many ordered pairs of positive integers ( x , y ) are there such that
x 1 + y 1 = 2 0 1 5 1 ?
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Nice solution. I know a little of Number Theory to understand your steps.
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Thanks. This is the standard approach to this type of problem; it's an application of what is known as Simon's Favorite Factoring Trick, (SFFT) .
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Nice Solution but please tell me how SFFT is used?
Use Simon's Favorite Factoring Trick
( x − 2 0 1 5 ) ( y − 2 0 1 5 ) = 2 0 1 5 2
As 2 0 1 5 2 = 3 1 2 × 1 3 2 × 5 2 ∴ number of divisors of 2 0 1 5 is 3 × 3 × 3 = 2 7
Did the same! :D
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Rewrite the equation as
x y x + y = 2 0 1 5 1 ⟹ x y − 2 0 1 5 x − 2 0 1 5 y = 0
⟹ ( x − 2 0 1 5 ) ( y − 2 0 1 5 ) = 2 0 1 5 2 .
Now 2 0 1 5 = 5 ∗ 1 3 ∗ 3 1 , and so 2 0 1 5 2 = 5 2 ∗ 1 3 2 ∗ 3 1 2 , which has ( 2 + 1 ) ( 2 + 1 ) ( 2 + 1 ) = 2 7 divisors. Now each of these divisors corresponds to a distinct solution pair ( ( x − 2 0 1 5 ) , ( y − 2 0 1 5 ) ) , which in turn corresponds to a distinct solution pair ( x , y ) . Thus there are 2 7 solution pairs.