Xin hay nhất nhé tus ^_^
`C =` $\frac{1}{3}$ `-` $\frac{3}{5}$ `+` $\frac{5}{7}$ `-` $\frac{7}{9}$ `+` $\frac{9}{11}$ `-` $\frac{11}{13}$ `+` $\frac{13}{15}$ `+` $\frac{11}{13}$ `-` $\frac{9}{11}$ `+...-` $\frac{1}{3}$
`C =` `(`$\frac{1}{3}$ `-` $\frac{1}{3}$`) + (`$\frac{3}{5}$ `-` $\frac{3}{5}$`) +...+ (`$\frac{11}{13}$ `-` $\frac{11}{13}$`) +` $\frac{13}{15}$
`C = 0 + 0 +...+ 0 +` $\frac{13}{15}$
`C = `$\frac{13}{15}$
