\(\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x4}\)
\(=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{20}=\frac{1}{60}+\frac{1}{60}+\frac{1}{60}+\frac{1}{60}=\frac{4}{60}=\frac{1}{15}\)
##
1/2.3+1/3.4+1/4.5+1/5.4
=1/2-1/3+1/3-1/4+1/4-1/5+1/20
=1/2-1/5+1/20
=3/10+1/20
=7/20
\(\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times4}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{20}\)
\(=\frac{1}{2}-\frac{1}{5}+\frac{1}{20}\)
\(=\frac{10}{20}-\frac{4}{20}+\frac{1}{20}\)
\(=\frac{7}{20}\)
HỌC TỐT
\(\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times4}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{20}\)
\(=\frac{1}{2}-\frac{1}{5}+\frac{1}{20}\)
\(=\frac{10}{20}-\frac{4}{20}+\frac{1}{20}\)
\(=\frac{7}{20}\)
