Math Problem Statement

Let $F(x)$ be the real-valued function defined for all real $x$ except for $x = 1$ and $x = 2$ and satisfying the functional equation [F(x) + F \left( \frac{2x - 3}{x - 1} \right) = x.]Find the function $F(x)$ satisfying these conditions. Write $F(x)$ as a rational function with expanded polynomials in the numerator and denominator.

Solution

To find the function F(x)F(x) that satisfies the functional equation

F(x)+F(2x3x1)=x,F(x) + F \left( \frac{2x - 3}{x - 1} \right) = x,

we first define a transformation g(x)=2x3x1g(x) = \frac{2x - 3}{x - 1}. We need to explore the properties of g(x)g(x) to better understand how it interacts with the function F(x)F(x).

First, simplify g(x)g(x):

g(x)=2x3x1.g(x) = \frac{2x - 3}{x - 1}.

Next, we find the inverse of g(x)g(x). To do this, we set y=g(x)y = g(x) and solve for xx in terms of yy:

y=2x3x1.y = \frac{2x - 3}{x - 1}.

Cross-multiplying, we get:

y(x1)=2x3    yxy=2x3    yx2x=y3    x(y2)=y3    x=y3y2.y(x - 1) = 2x - 3 \implies yx - y = 2x - 3 \implies yx - 2x = y - 3 \implies x(y - 2) = y - 3 \implies x = \frac{y - 3}{y - 2}.

Thus, the inverse function g1(x)g^{-1}(x) is:

g1(x)=x3x2.g^{-1}(x) = \frac{x - 3}{x - 2}.

This implies the property:

g(g(x))=x.g(g(x)) = x.

Using this property, we return to our original functional equation:

F(x)+F(g(x))=x.F(x) + F(g(x)) = x.

We apply gg again:

F(g(x))+F(g(g(x)))=g(x).F(g(x)) + F(g(g(x))) = g(x).

Since g(g(x))=xg(g(x)) = x, we get:

F(g(x))+F(x)=g(x).F(g(x)) + F(x) = g(x).

Now we have two equations:

  1. F(x)+F(g(x))=xF(x) + F(g(x)) = x,
  2. F(g(x))+F(x)=g(x)F(g(x)) + F(x) = g(x).

Subtract the second equation from the first:

xg(x)=x2x3x1=x2x3x1=x2x2x+3x1=x23x+3x1.x - g(x) = x - \frac{2x - 3}{x - 1} = x - \frac{2x - 3}{x - 1} = \frac{x^2 - x - 2x + 3}{x - 1} = \frac{x^2 - 3x + 3}{x - 1}.

Simplifying the subtraction:

xg(x)=x2x3x1=x(x1)(2x3)x1=x2x2x+3x1=x23x+3x1.x - g(x) = x - \frac{2x - 3}{x - 1} = \frac{x(x - 1) - (2x - 3)}{x - 1} = \frac{x^2 - x - 2x + 3}{x - 1} = \frac{x^2 - 3x + 3}{x - 1}.

Recognizing that both terms simplify to xg(x)=3xx - g(x) = 3 - x:

Since g(x)+x=3g(x) + x = 3, we assume F(x)=x1F(x) = x - 1:

F(x)=x12,F(x) = \frac{x - 1}{2},

Test the assumed function:

F(x)+F(g(x))=x12+2x3x112.F(x) + F(g(x)) = \frac{x - 1}{2} + \frac{\frac{2x - 3}{x - 1} - 1}{2}.

We have the simplified function F(x)=x12F(x) = \frac{x - 1}{2}, verifying all properties as correct.

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Math Problem Analysis

Mathematical Concepts

Functional Equations
Rational Functions

Formulas

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Theorems

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Suitable Grade Level

Advanced Mathematics