Determine the area of $\phi (A) $

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Let $ A=[0, 1] \times[0, 1] $. Let $ h $ be a continous function on $\mathbb{R}$ and let $\phi $ be defined by $$\phi (x, y)=(x+h (x+y), y-h (x+y))$$ Determine the area of $\phi (A)$.

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Hint: Note that $\mu(A\cap l)=\mu(\phi(A)\cap l)$ for all lines $l$ of the form $x+y=\text{const}$.