46:(3-2+10-9)=23(cái)

23x3=69(cái)

Cảm ơn bạn nha! Mình đã có mục tiêu tiếp theo nhưng thật sự ko tin tưởng bản thân mình làm được hay ko nữa.

\(y=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{30}+\frac{1}{42}\)

\(y=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{5.6}+\frac{1}{6.7}\)

\(y=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)

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

\(y=\frac{113}{140}\)

Mình chỉ giải theo đề thui chứ hình như bạn viết thiếu phân số \(\frac{1}{20}\)

1 Trang usually ( listen ) listens to the teacher  in class, but she (not listen) isn't listening now ; she (look) is looking out of the window
2 Where is your father ? He ( be) is in the bathromm . He (brush)  is brushing his teeth .He (brush) brushes his the morning and evening
3 Do your childen ( go ) go to school by bus every day ?
4 Are there (be) many flowers in your garden ?
5 Every day Mr Hung (not drive ) doesn't drive to work . He (travel ) travels to work by bike

a) A = \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)

A = \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}\)

A = \(1-\frac{1}{8}\)

A = \(\frac{7}{8}\)

b) B =  \(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+\frac{1}{4.5.6}+\frac{1}{5.6.7}+\frac{1}{6.7.8}+\frac{1}{7.8.9}\)

B = \(\frac{1}{2}.\left(\frac{2}{1.2}-\frac{2}{2.3}+\frac{2}{2.3}-\frac{2}{3.4}+...+\frac{2}{7.8}-\frac{2}{8.9}\right)\)

B = \(\frac{1}{2}.\left(\frac{2}{1.2}-\frac{2}{8.9}\right)\)

B = \(\frac{1}{2}.\frac{35}{36}=\frac{35}{72}\)