$X$ be a projective curve of $\mathbb{P}^n$, $P$ is not $X$ and let $f:X\rightarrow \mathbb{P}^{n-1}$ be the projection from $P$. When I read Hartshorne book, I see that if $f$ is insepable (i.e the function field of $X$ is inseparable extension of the function field of image), then for any $Q$ in $X$, the tangent line $L_Q$ at $X$ passes through $P$. But I can't understnad this fact.... Help me...
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