The linear approximation is given by the equation:
L(x,y) = f(a,b) + f_x(a,b)(x-a) + f_y(a,b)(y-b)
where f(a,b) is the value of the function at the point (a,b), f_x(a,b) is the partial derivative of the function with respect to x evaluated at (a,b), and f_y(a,b)
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