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**lllll** $\displaystyle f(x,y) = \left\{ \begin{array}{rcl}

k(1-y) & \mbox{for} & 0 \leq x \leq y \leq 1 \\

0 & \mbox{for} & \mbox{other}

\end{array}\right. $

for $\displaystyle 0 \leq x \leq y \leq 1$ is it $\displaystyle 0 \leq x \leq \ \mbox{and} \ x \leq y \leq 1$ which would give:

$\displaystyle \int_{0}^{1} \int_{x}^{1} k(1-y) \ dy \ dx$ ?

and does it matter if it's $\displaystyle 0 \leq x \leq 1 \ \mbox{and} \ x \leq y \leq 1$ or $\displaystyle 0 \leq y \leq \ 1 \ \mbox{and} \ 0 \leq x \leq y$ ?