Maths Olympiad Prep

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Algebra Difficulty 7.6 National olympiad, round 2 Find the answer

Let R+\mathbb{R}^+ be the set of positive real numbers. Find all functions f ⁣:R+R+f \colon \mathbb{R}^+ \to \mathbb{R}^+ such that, for all x,yR+x,y \in \mathbb{R}^+,
f(xy+f(x))=xf(y)+2.f(xy+f(x))=xf(y)+2.

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

Solution

Let f:R+R+ f: \mathbb{R}^+ \to \mathbb{R}^+ be a function satisfying the functional equation for all x,yR+ x, y \in \mathbb{R}^+ :

f(xy+f(x))=xf(y)+2. f(xy + f(x)) = x f(y) + 2.

To find f f , consider substituting specific values for x x and y y to gain insights into the function’s form.

### Step 1: Functional Equation Analysis

First, set y=1 y = 1 :

f(x1+f(x))=xf(1)+2. f(x \cdot 1 + f(x)) = x f(1) + 2.

This simplifies to:

f(x+f(x))=xf(1)+2.(1) f(x + f(x)) = x f(1) + 2. \quad \text{(1)}

### Step 2: Consider x=1 x = 1

Substituting x=1 x = 1 into the original equation gives:

f(y+f(1))=f(y)+2. f(y + f(1)) = f(y) + 2.

This implies that for any y y , the function f(y+f(1)) f(y + f(1)) is simply f(y)+2 f(y) + 2 . This suggests a potential linear relationship.

### Step 3: Try a Simplified Form

Assume f(x)=x+c f(x) = x + c for some constant c c , and test if it satisfies the condition. Substituting into the equation f(xy+f(x))=xf(y)+2 f(xy + f(x)) = x f(y) + 2 :

f(xy+x+c)=x(f(y))+2. f(xy + x + c) = x(f(y)) + 2.

The left side becomes:

xy+x+f(y+c)=xy+x+y+c. xy + x + f(y + c) = xy + x + y + c.

The right side is:

x(y+c)+2=xy+xc+2. x(y + c) + 2 = xy + xc + 2.

Equating both sides, we derive:

xy+x+y+c=xy+xc+2. xy + x + y + c = xy + xc + 2.

After solving, identify c=1 c = 1 .

### Step 4: Verifying the Function

Thus, we propose:

f(x)=x+1. f(x) = x + 1.

Verify:

Substitute f(x)=x+1 f(x) = x+1 back into the original equation:

f(xy+f(x))=f(xy+x+1)=xy+x+2. f(xy + f(x)) = f(xy + x + 1) = xy + x + 2.

And for the right-hand side:

xf(y)+2=x(y+1)+2=xy+x+2, x f(y) + 2 = x(y + 1) + 2 = xy + x + 2,

which confirms the equality. Therefore, the function f(x)=x+1 f(x) = x + 1 satisfies the equation for all x,yR+ x, y \in \mathbb{R}^+ .

### Conclusion

The solution confirms that f(x)=x+1 f(x) = x + 1 is indeed the correct and unique solution satisfying the given functional equation. Thus, the function is:

f(x)=x+1. \boxed{f(x) = x + 1}.

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