13. Tính tổng sau : a) 10 + 11/19 + 8/19 b) -13/24 + 2/7 + -11/24 c) 10/8 + -24/48 + -105/120

`a)``10``+`$\frac{11}{19}$`+`$\frac{8}{19}$
  `=``10``+``(`$\frac{11}{19}$`+`$\frac{8}{19}$`)`
  `=``10``+``1`
  `=``11`

`b)`$\frac{-13}{24}$`+` $\frac{2}{7}$`+`$\frac{-11}{24}$
 `=``(`$\frac{-13}{24}$`+`$\frac{-11}{24}$`)``+`$\frac{2}{7}$
 `=``(-1)``+`$\frac{2}{7}$
 `=`$\frac{-7}{7}$ `+`$\frac{2}{7}$
 `=`$\frac{-5}{7}$ 

`c)`$\frac{10}{8}$`+`$\frac{-24}{48}$`+` $\frac{-105}{120}$ 
  `=`$\frac{60}{48}$ `+``$\frac{-24}{48}$`+` $\frac{-105}{120}$ 
  `=`$\frac{3}{4}$ `+` $\frac{-105}{120}$ 
  `=`$\frac{90}{120}$ `+` $\frac{-105}{120}$ 
  `=`$\frac{-15}{120}$ 
  `=`$\frac{-1}{8}$