We want to show that $x \cdot x = x$.
Since $f$ is a ring homomorphism, it preserves multiplication. Thus, $f(1 \cdot 1) = f(1) \cdot f(1)$.
Since $1 \cdot 1 = 1$, we have $f(1) = f(1) \cdot f(1)$.
Substituting $x = f(1)$, we get $x = x \cdot x$, which is what we
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