# Dedicated to Comrade Otto Bretscher.

Calculus Level 5

Let $$S$$ be the surface: $z=x(1-x)y(1-y)$

Such that ,

$$0\leq x,y\leq 1$$

Evaluate the integral :

$\int \int_{S} x\mathbf{k}.d\mathbf{A}$ where , $$d\mathbf{A}$$ is the upward-pointing normal vector

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