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Algebra Difficulty 6.5 National olympiad Find the answer

Find all functions ff and gg defined from R>0\mathbb{R}_{>0} to R>0\mathbb{R}_{>0} such that for all x,y>0x, y > 0 the two equations hold
(f(x)+y1)(g(y)+x1)=(x+y)2 (f(x) + y - 1)(g(y) + x - 1) = {(x + y)}^2
(f(x)+y)(g(y)+x)=(x+y+1)(yx1) (-f(x) + y)(g(y) + x) = (x + y + 1)(y - x - 1)

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the given functional equations, we need to find functions f f and g g that satisfy the following two conditions for all x,y>0 x, y > 0 :

1.
(f(x)+y1)(g(y)+x1)=(x+y)2 (f(x) + y - 1)(g(y) + x - 1) = (x + y)^2

2.
(f(x)+y)(g(y)+x)=(x+y+1)(yx1) (-f(x) + y)(g(y) + x) = (x + y + 1)(y - x - 1)

### Step 1: Analyze the First Equation

Consider the first equation:

(f(x)+y1)(g(y)+x1)=(x+y)2 (f(x) + y - 1)(g(y) + x - 1) = (x + y)^2

Assume f(x)=x+c f(x) = x + c and g(y)=y+d g(y) = y + d for some constants c c and d d . Substitute these into the equation:

(x+c+y1)(y+d+x1)=(x+y)2 (x + c + y - 1)(y + d + x - 1) = (x + y)^2

This simplifies to:

(x+y+c1)(x+y+d1)=(x+y)2 (x + y + c - 1)(x + y + d - 1) = (x + y)^2

By comparing terms, we get:

x+y+c1=x+y+d1=x+y x + y + c - 1 = x + y + d - 1 = x + y

Thus, c=1 c = 1 and d=1 d = 1 .

### Step 2: Verify with the Second Equation

Now, substitute f(x)=x+1 f(x) = x + 1 and g(y)=y+1 g(y) = y + 1 into the second equation:

(f(x)+y)(g(y)+x)=(x+y+1)(yx1) (-f(x) + y)(g(y) + x) = (x + y + 1)(y - x - 1)

Substituting f f and g g , we have:

((x+1)+y)((y+1)+x)=(x+y+1)(yx1) (-(x + 1) + y)((y + 1) + x) = (x + y + 1)(y - x - 1)

This further simplifies to:

(yx1)(x+y+1)=(x+y+1)(yx1) (y - x - 1)(x + y + 1) = (x + y + 1)(y - x - 1)

Both sides of the equation are identical, confirming our solution satisfies the second equation.

### Conclusion

The functions f(x)=x+1 f(x) = x + 1 and g(y)=y+1 g(y) = y + 1 satisfy both functional equations. Therefore, the solution is:

f(x)=x+1 and g(y)=y+1 \boxed{f(x) = x + 1 \text{ and } g(y) = y + 1}

These functions are defined and satisfy the given equations for all x,y>0 x, y > 0 .

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.