Question

Câu 3: \(\frac{3}{4x5}+\frac{3}{5x6}+\frac{3}{6x7}+...+\frac{3}{18x19}+\frac{3}{19x20}\)

Answer 1

\(\frac{3}{4.5}+\frac{3}{5.6}+\frac{3}{6.7}+.....+\frac{2}{19.20}\)

\(=3.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+....+\frac{1}{19.20}\right)\)

\(=3.\left(\frac{1}{4}-\frac{1}{20}\right)\)

\(=3.\frac{1}{5}=\frac{3}{5}\)

Answer 2

giúp mik với mik cần  gấp

Answer 3

Đặt \(A=\frac{3}{4.5}+\frac{3}{5.6}+\frac{3}{6.7}+..+\frac{3}{19.20}\)

\(A=3\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{19.20}\right)\)

\(A=3\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(A=3\left(\frac{1}{4}-\frac{1}{20}\right)\)

\(A=\frac{3}{5}\)

Ủng hộ mk nha !!! ^_^

Answer 4

     \(\frac{3}{4.5}+\frac{3}{5.6}+\frac{3}{6.7}+...+\frac{3}{18.19}+\frac{3}{19.20}\)

\(=3\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{18.19}+\frac{1}{19.20}\right)\)

\(=3\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{18}-\frac{1}{19}+\frac{1}{19}-\frac{1}{20}\right)\)

\(=3\left(\frac{1}{4}-\frac{1}{20}\right)\)

\(=3.\frac{4}{20}=\frac{12}{20}=\frac{3}{5}\)