A = [tex]\frac{1}{3.10} + \frac{1}{10.17} + \frac{1}{17.24} +...+ \frac{1}{73.80} - \frac{1}{2.9} - \frac{1}{9.16} -...-\frac{1}{23.30}[/tex]
Làm ơn các bn giải chi tiết hộ mik nha !!!! Mik cần gấp :r2:r2:r2
$A=\dfrac{1}{7}(\dfrac{7}{3.10}+\dfrac{7}{10.17}+\dfrac{7}{17.24}+...+\dfrac{7}{73.80}-\dfrac{7}{2.9}-\dfrac{7}{9.16}-...-\dfrac{7}{23.30})
\\=\dfrac{1}{7}(\dfrac{1}{3}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{17}+\dfrac{1}{17}-\dfrac{1}{24}+...+\dfrac{1}{73}-\dfrac{1}{80}-\dfrac{1}{2}+\dfrac{1}{9}-\dfrac{1}{9}+\dfrac{1}{16}-...-\dfrac{1}{23}+\dfrac{1}{30})
\\=\dfrac{1}{7}(\dfrac{1}{3}-\dfrac{1}{80}-\dfrac{1}{2}+\dfrac{1}{30})=\dfrac{1}{7}.\dfrac{-7}{48}=\dfrac{-1}{48}$