a. $(3,9 + \frac{1}{2}.2,6):13$
$=\left ( \frac{39}{10}+\frac{1}{2}.\frac{13}{5} \right ):13$
$=\left ( \frac{39}{10}+\frac{26}{10} \right ).\frac{1}{13}$
$=\frac{65}{10}.\frac{1}{13}=\frac{1}{2}$
$3,9 + \frac{1}{2}.2,6 : 13$
$=\frac{39}{10}+\frac{1}{2}.\frac{13}{5}.\frac{1}{13}$
$=\frac{39}{10}+\frac{1}{10}=\frac{40}{10}=4$
=> $(3,9 + \frac{1}{2}.2,6):13\neq 3,9 + \frac{1}{2}.2,6 : 13$
b. $\frac{5}{4}-\left ( \frac{1}{4} \right )^{2}=\frac{20}{16}-\frac{1}{16}=\frac{19}{16}$
$\left ( \frac{5}{4}-\frac{1}{4} \right )^{2}=1^{2}=1$
=>$\frac{5}{4} - \left ( \frac{1}{4} \right )^{2}\neq \left ( \frac{5}{4} - \frac{1}{4} \right )^{2}$
c. $\frac{28}{9}.0,7+\frac{28}{9}.0,5=\frac{28}{9}.(0,7+0,5)$
=> $\frac{28}{9}.0,7+\frac{28}{9}.0,5= \frac{28}{9}.(0,7 + 0,5)$
d. $\frac{36}{13}:4+\frac{36}{13}:9$
$=\frac{36}{13}.\frac{1}{4}+\frac{36}{13}.\frac{1}{9}$
$=\frac{36}{13}.\left ( \frac{1}{4}+\frac{1}{9} \right )$
$=\frac{36}{13}.\frac{13}{36}=1$
$\frac{36}{13}:(4 +9)$
$=\frac{36}{13}:13$
$= \frac{36}{13}.\frac{1}{13}$
$=\frac{36}{169}$
=>$\frac{36}{13}:4+\frac{36}{13}:9\neq \frac{36}{13}:(4 +9)$