In the ring of polynomials $F[t]$, every ideal is principal

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Let

$$\langle a \rangle=\bigcap_{ a \in I} I$$

$$ \langle a \rangle =\{ Ira + na, r\in R,n \in Z\}$$

What is unclear is why$$ I=\langle 0 \rangle$$ proves that I is a principal ideal. My definition of principal ideal is given.