Hint: Analyse the denominator First ^_^

Algebra Level 2

Let ( x , y ) N (x,y) \in \mathbb{N} such that ( x , y ) (x,y) is the solution of

1 x + 1 + 1 y 1 = 5 6 \frac{1}{x+1} + \frac{1}{y-1} = \frac{5}{6} .

Find x + y x+y .

2 3 4 6 5

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3 solutions

Parth Panchal
Apr 24, 2015

Mr Paul made us all fool by giving wrong hint :P no offence ! just solve the numerator which is x+y on the RHS numerator is 5 as LHS=RHS --> x+y=5

Shazid Reaj
Apr 10, 2015

5/6 = (3+2)/6 =3/6 + 2/6 =1/2 + 1/3 = 1/(1+1) + 1/(4-1)

let, x = 1 and y = 4 so, x+y = 5

Paul Ryan Longhas
Mar 26, 2015

1 > 5 6 = > x + 1 2 1 > \frac{5}{6} => x+1 \geq 2 and y 1 2 y-1 \geq 2 .

One of 1 x + 1 \frac{1}{x+1} and 1 y 1 \frac{1}{y-1} is at least half of 5 6 \frac{5}{6} , so either x + 1 12 5 = 2.4 x+1 \leq \frac{12}{5}=2.4 or y 1 12 5 = 2.4 y-1 \leq \frac{12}{5}=2.4 .

Consider the two cases:

Case 1, x + 1 = 2 = > x = 1 > y 1 = 3 = > y = 4 x+1 = 2 => x=1 ---> y-1 = 3 => y =4 .

Case 2, y 1 = 2 = > y = 3 > x + 1 = 3 = > x = 2 y-1 = 2 => y = 3 ---> x+1 =3 => x =2 .

In either case, x + y = 5 x+y = 5 .

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