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So we can conclude that $c_{i j}= c (A) $  5. 6.  6.14. Justify the following items which prove: 
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(e) Therefore, for any $(a,b)$, $f(a,b)$ is defined and is neither positive nor negative, so it must be $0$.  7.  -6.44. Justify the following steps to prove that if $f$ is integrable on $R_2$ and $g$ is a  continuous function with $0 \leq g \leq f$ then $g$ is integrable on $R_2$.  (a) $\int_{D(n)} g dA$ exsits  (b) 0\leq $\int_{D(n)} g dA$ \leq $\int_{D(n)} f dA$  (c) The numbers $\int_{D(n)} g dA$ are an increasing sequence bounded above.  (d)