\(B=\frac{3}{2.7}+\frac{3}{7.12}+\frac{3}{12.17}+...+\frac{3}{87.92}\)
\(\Leftrightarrow\frac{5}{3}B=\frac{5}{2.7}+\frac{5}{7.12}+\frac{5}{12.17}+...+\frac{5}{87.92}\)
\(\Leftrightarrow\frac{5}{3}B=\frac{7-2}{2.7}+\frac{12-7}{7.12}+\frac{17-12}{12.17}+...+\frac{92-87}{87.92}\)
\(\Leftrightarrow\frac{5}{3}B=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{17}+...+\frac{1}{87}-\frac{1}{92}\)
\(\Leftrightarrow\frac{5}{3}B=\frac{1}{2}-\frac{1}{92}\)
\(\Leftrightarrow\frac{5}{3}B=\frac{46}{92}-\frac{1}{92}\)
\(\Leftrightarrow\frac{5}{3}B=\frac{45}{92}\)
\(\Leftrightarrow B=\frac{45}{92}\times\frac{3}{5}\)
\(\Leftrightarrow B=\frac{27}{92}\)
CTV làm màu vc :)
\(B=\frac{3}{2.7}+\frac{3}{7.12}+\frac{3}{12.17}+...+\frac{3}{87.92}\)
\(B=\frac{3}{5}.\left(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{17}+...+\frac{1}{87}-\frac{1}{92}\right)\)
\(B=\frac{3}{5}.\left(\frac{1}{2}-\frac{1}{92}\right)\)
\(B=\frac{3}{5}.\frac{45}{92}\)
\(B=\frac{27}{92}\)
