\(\left(2746-x\right)-258=1360\)

\(2746-x=1360+258\)

\(2746-x=1618\)

\(x=2746-1618\Rightarrow x=1128\)

\(a.1\frac{1}{3}+2\frac{1}{2}\)

\(=\frac{4}{3}+\frac{5}{2}\)

\(=\frac{8}{6}+\frac{15}{6}\)

\(=\frac{23}{6}\)

\(b.3\frac{2}{5}-1\frac{1}{7}\)

\(=\frac{17}{5}-\frac{8}{7}\)

\(=\frac{119}{35}-\frac{40}{35}\)

\(=\frac{79}{35}\)

\(c.3\frac{1}{2}.1\frac{1}{7}\)

\(=\frac{7}{2}.\frac{8}{7}\)

\(=4\)

\(d.4\frac{1}{6}:2\frac{1}{3}\)

\(=\frac{25}{6}:\frac{7}{3}\)

\(=\frac{25}{6}.\frac{3}{7}\)

\(=\frac{25}{14}\)

\(1+\frac{1}{1+2}+\frac{1}{1+2+3}+\frac{1}{1+2+3+4}+...+\frac{1}{1+2+3+...+2015}\)

\(=\frac{1}{\left(1+0\right).2:2}+\frac{1}{\left(1+2\right).2:2}+\frac{1}{\left(1+3\right).3:2}+\frac{1}{\left(1+4\right).4:2}+...+\frac{1}{\left(1+2015\right).2015:2}\)

\(=\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+...+\frac{2}{2015.2016}\)

\(=2.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{2015.2016}\right)\)

\(=2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{2015}-\frac{1}{2016}\right)\)

\(=2.\left(1-\frac{1}{2016}\right)\)

\(=2.\frac{2015}{2016}=\frac{2015}{1008}\)

dãy 1: 40, 74, 136, 250, 460

ở dãy 1 thì số đứng sau bằng tổng hai số đứng trước 

ta có 5 số tiếp theo la 40,74, 136,...

98/99 nha bạn nhớ tk mình và kb nha

=1/1-1/99

=98/99

\(a.\)\(1\frac{2}{3}:\frac{2}{3}-\frac{3}{4}\cdot\frac{2}{3}+5\frac{3}{7}\)

\(=\frac{5}{3}:\frac{2}{3}-\frac{3}{4}\cdot\frac{2}{3}+\frac{38}{7}\)

\(=\frac{5}{3}\cdot\frac{3}{2}-\frac{3}{4}\cdot\frac{2}{3}+\frac{38}{7}\)

\(=\frac{5}{2}-\frac{1}{2}+\frac{38}{7}\)

\(=\frac{4}{2}+\frac{38}{7}\)

\(=2+\frac{38}{7}\)

\(=\frac{14}{7}+\frac{38}{7}\)

\(=\frac{52}{7}\)

\(b.1\frac{1}{3}-1\frac{1}{4}:1\frac{1}{2}+2\frac{3}{4}\cdot3\frac{2}{3}\)

\(=\frac{4}{3}-\frac{5}{4}:\frac{3}{2}+\frac{11}{4}\cdot\frac{11}{3}\)

\(=\frac{4}{3}-\frac{5}{4}\cdot\frac{2}{3}+\frac{11}{4}\cdot\frac{11}{3}\)

\(=\frac{4}{3}-\frac{5}{6}+\frac{121}{12}\)

\(=\frac{16}{12}-\frac{10}{12}+\frac{121}{12}\)

\(=\frac{6}{12}+\frac{121}{12}\)

\(=\frac{127}{12}\)

\(c.7\cdot\frac{2}{3}-\frac{2}{5}:\frac{1}{2}-\frac{2}{3}\)

\(=7\cdot\frac{2}{3}-\frac{2}{5}\cdot\frac{2}{1}-\frac{2}{3}\)

\(=7\cdot\frac{2}{3}-\frac{4}{5}-\frac{2}{3}\)

\(=\frac{14}{3}-\frac{4}{5}-\frac{2}{3}\)

\(=\frac{70}{15}-\frac{12}{15}-\frac{10}{15}\)

\(=\frac{58}{15}-\frac{10}{15}\)

\(=\frac{48}{15}=\frac{16}{5}\)

\(\frac{5}{3}:\frac{2}{3}-\frac{3}{4}\cdot\frac{2}{3}+\frac{38}{7}\)

\(\frac{5}{2}-\frac{1}{2}+\frac{38}{7}\)

\(2+\frac{38}{7}\)

\(\frac{52}{7}\)

MN giúp mk với

1/1+2+1/1+2+3+1/1+2+3+4+1/1+2+3+4+5

=1/3+1/6+1/10+1/15

lấy mẫu số chung là 30,ta có:

10/30+5/6+3/10+2/30

=20/30=2/3