5Question: Let $ f(x) $ be a polynomial such that $ f(x^2 + 2) = x^4 + 4x^2 + 4 $. Find $ f(x^2 - 2) $.

Title: Solving Functional Polynomial Equations: Find $ f(x^2 - 2) $ Given $ f(x^2 + 2) = x^4 + 4x^2 + 4 $

Meta Description:
Explore algebraic reasoning and polynomial substitution with $ f(x^2 + 2) = x^4 + 4x^2 + 4 $. Learn how to determine $ f(x^2 - 2) $ step-by-step.

Understanding the Context

Introduction

Functional equations involving polynomials often reveal deep structure when approached systematically. One such problem asks:

> Let $ f(x) $ be a polynomial such that $ f(x^2 + 2) = x^4 + 4x^2 + 4 $. Find $ f(x^2 - 2) $.

At first glance, this may seem abstract, but by applying substitution and polynomial identification, we unlock a clear path forward. This article guides you through solving this elegant functional equation and computing $ f(x^2 - 2) $.

Key Insights

Step 1: Analyze the Given Functional Equation

We are given:

$$
f(x^2 + 2) = x^4 + 4x^2 + 4
$$

Notice that the right-hand side is a perfect square:

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Final Thoughts

$$
x^4 + 4x^2 + 4 = (x^2 + 2)^2
$$

So the equation becomes:

$$
f(x^2 + 2) = (x^2 + 2)^2
$$

This suggests that $ f(u) = u^2 $, where $ u = x^2 + 2 $. Since this holds for infinitely many values (and both sides are polynomials), we conclude:

$$
f(u) = u^2
$$

That is, $ f(x) = x^2 $ is the polynomial satisfying the condition.

Step 2: Compute $ f(x^2 - 2) $

Now that we know $ f(u) = u^2 $, substitute $ u = x^2 - 2 $:

$$
f(x^2 - 2) = (x^2 - 2)^2
$$