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Let $f \in k[x_0,\ldots,x_n]$ be a homogeneous irreducible polynomial.And let $X=\mathbb{P}^n - \mathcal{Z}(f)$.It is known that $X$ is isomorphic to an affine variety. Is it true that its coordinate ring $A(X)$ is isomorphic to $k[x_0,\ldots,x_n]_{(f)}$ ?

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An affine variety is quasi-projective. (If $X$ is closed in $\mathbf{A}^n$, then it is a closed of an open of $\mathbf{P}^n$. Thus, it is quasi-projective.) –  Harry Nov 6 '12 at 13:13
    
The notation for the zero locus of $f$ in $\mathbf{P}^n$ is $V_+(f)$ or $Z_+(f)$ in general. –  Harry Nov 6 '12 at 13:14
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If that is your notation for the degree-zero part of the localization of $k[x_0,\ldots,x_n]$ in $f$, then this is true. A way to see it is by applying the $d$-uple (veronese) embedding. That way, you can assume $f$ to be linear, i.e. you may assume that it is one of the $x_i$. In this case, your statement is just the well-known fact that the standard open sets $U_i=\mathbb{P}^n\setminus\mathcal{Z}(x_i)$ are isomorphic to $\mathbb{A}^n$.

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