Question

Tính nhanh:

\(\frac{1}{20}\)+\(\frac{1}{30}\)+\(\frac{1}{42}\)

lm nhanh giúp mk vs nha!!!!!

Answer 1

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)

\(=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}\)

\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\frac{1}{4}-\frac{1}{7}\)

\(=\frac{7}{28}-\frac{4}{28}\)

\(=\frac{3}{28}\)

    Chúc bạn học tốt

Answer 2

Ta có : 

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)

\(=\)\(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)

\(=\)\(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\)\(\frac{1}{4}-\frac{1}{7}\)

\(=\)\(\frac{3}{28}\)

Chúc bạn học tốt ~ 

Answer 3

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)

\(=\frac{1}{4x5}+\frac{1}{5x6}+\frac{1}{6x7}\)

\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\frac{1}{4}-\frac{1}{7}\)

\(=\frac{3}{28}\)

Answer 4

\(=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}\)

\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

\(=\frac{1}{4}-\frac{1}{7}\)

\(=\frac{3}{28}\)