ta có :
M=\(\frac{1}{2.15}+\frac{1}{15.3}+\frac{1}{3.21}+....+\frac{6}{87.90}\)
\(\frac{1}{6}\)M=\(\frac{1}{6.2.15}+\frac{1}{15.3.6}+\frac{1}{6.3.21}+...+\frac{6}{6.87.90}\)
\(\frac{1}{6}\)M=\(\frac{1}{12.15}+\frac{1}{15.18}+\frac{1}{18.21}+...+\frac{1}{87.90}\)
\(\frac{1}{6}\)M=\(\frac{1}{3}\left(\frac{3}{12.15}+\frac{3}{15.18}+\frac{3}{18.21}+...+\frac{3}{87.90}\right)\)
\(\frac{1}{6}\)M=\(\frac{1}{3}\)(\(\frac{1}{12}-\frac{1}{15}+\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{87}-\frac{1}{90}\))
\(\frac{1}{6}\)M=\(\frac{1}{3}\left(\frac{1}{12}-\frac{1}{90}\right)\)
\(\frac{1}{6}\)M=\(\frac{1}{3}.\frac{13}{180}\)
\(\frac{1}{6}\)M=\(\frac{13}{540}\)
M=\(\frac{13}{540}:\frac{1}{6}\)
M=\(\frac{13}{90}\)
Vậy M=\(\frac{13}{90}\)
thôi , xin lỗi đã làm mất thời gian của mọi người nha
mình biết làm rùi
