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$K$ is field. $a_1$,$a_2$ elements of $K$. Show that $(x_1-a_1,x_2-a_2)$ is a maximal ideal of $K[x_1,x_2]$.

$K[x_1,x_2]$ is UFD so if $K[x_1,x_2]/(x_1-a_1,x_2-a_2)$ is field then $(x_1-a_1,x_2-a_2)$ is maximal ideal.

If I can show that $K[x_1,x_2]/(x_1-a_1,x_2-a_2)$ isomorphic to $K$, we can verify that $(x_1-a_1,x_2-a_2)$ maximal ideal of $K[x_1,x_2]$.

thanks for helps and comments.

user26857
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spectralmath
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    Hint: $$\phi:K[x_1,x_2]/\langle x_1-a,x_2-b\rangle \to K;,;\phi(f(x,y):=f(a,b);\ldots$$ – DonAntonio Nov 03 '13 at 22:06
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    $K[x,y]/(x-a_1, y-a_2)$ is just $K[x,y]$ modulo the relations $x=a_1$ and $y=a_2$. Of course it is isomorphic to $K$. You can fill in the details. – TBrendle Nov 03 '13 at 22:08

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Consider the following map (slightly modified the suggestion of @DonAntonio in comments): $$\phi:K[x,y]\to K,\quad \phi(f(x,y)):=f(a,b)$$ Then show that its kernel is just the ideal $(x-a,\,y-b)$ and its image contains $1$, so it is surjective, then use the first isomorphism theorem.

Berci
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