C7
a)Ta có ($\frac{3}{4}$ - $\frac{3}{5}$ + $\frac{3}{7}$ + $\frac{3}{13}$ ): ($\frac{11}{7}$ + $\frac{11}{13}$ + $\frac{11}{4}$ - $\frac{11}{5}$ )
= [3($\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{7}$ + $\frac{1}{13}$ )] : [11($\frac{1}{7}$ + $\frac{1}{13}$ + $\frac{1}{4}$ - $\frac{1}{5}$ )]
= $\frac{3}{11}$
b) Ta có ($\frac{11}{7}$ + $\frac{11}{3}$ ) : ( $\frac{5}{7}$ + $\frac{15}{9}$ )
= ($\frac{33}{21}$ + $\frac{77}{21}$ ) : ($\frac{45}{63}$ + $\frac{105}{63}$ )
=$\frac{110}{21}$ : $\frac{50}{21}$ =$\frac{11}{5}$
