Happy new year 2021

Given that x x and y y are non-negative integers and x + y + y x = 2021 x+y+yx =2021 , how many ordered-pair ( x , y ) (x,y) solutions are there?


The answer is 8.

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1 solution

Chew-Seong Cheong
Dec 30, 2020

Given that:

x + y + y x = 2021 x y + x + y + 1 = 2022 ( x + 1 ) ( y + 1 ) = 2022 = 2 × 3 × 337 \begin{aligned} x + y + yx & = 2021 \\ xy + x + y + 1 & = 2022 \\ (x+1)(y+1) & = 2022 = 2 \times 3 \times 337 \end{aligned}

Note that 2 2 , 3 3 , and 337 337 are primes. Therefore 2022 = 2 1 × 3 1 × 33 7 1 2022 = 2^\red 1 \times 3^\red 1 \times 337^\red 1 has ( 1 + 1 ) ( 1 + 1 ) ( 1 + 1 ) = 8 (\red 1 +1) (\red 1 +1) (\red 1 +1) = 8 divisors of x + 1 x+1 and y + 1 y+1 . Hence there are 8 \boxed 8 ordered-pair ( x , y ) (x,y) solutions.

H a p p y N e w Y e a r \Large \purple {Happy \ New \ Year}

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