ta có : (ghi lại đề)

=6+12+18+24+30/3+6+9+12+15

=2*(3/3+6/6+9/9+12/12+15/15)

=2*(1+1+1+1+1)

=2*5=10

chúc main học tốt nhé

hi

bấm máy tính

tự vận đông đê

\(a,165:11\cdot999+85\cdot999=15\cdot999+85\cdot999=999\left(15+85\right)=999\cdot100=99900\)

\(b,25\cdot4+25:5-4\left(30-5\right)-5=25\cdot4+5-4\cdot25-5=\left(25\cdot4-4\cdot25\right)+\left(5-5\right)=0+0=0\)

Cảm ơn.

Bài 3:

a: Ta có: \(23\left(42-x\right)=23\)

\(\Leftrightarrow42-x=1\)

hay x=41

b: Ta có: 15(x-3)=30

nên x-3=2

hay x=5

Bài 1: 

a: 32+89+68=100+89=189

b: 64+112+236=300+112=412

c: \(1350+360+650+40=2000+400=2400\)

a) \(\dfrac{-5}{9}-\dfrac{4}{15}+\dfrac{2}{9}-\dfrac{11}{15}\)

\(=\left(\dfrac{-5}{9}+\dfrac{2}{9}\right)-\left(\dfrac{4}{15}+\dfrac{11}{15}\right)\)

\(=\dfrac{-3}{9}-\dfrac{15}{15}\)

\(=-\dfrac{1}{3}-1\)

\(=-\dfrac{4}{3}\)

b) \(\dfrac{-8}{13}-\dfrac{7}{16}+\dfrac{21}{13}-\dfrac{1}{6}\)

\(=\left(\dfrac{-8}{13}+\dfrac{21}{13}\right)-\dfrac{7}{16}-\dfrac{1}{6}\)

\(=\dfrac{13}{13}-\dfrac{29}{48}\)

\(=1-\dfrac{29}{48}\)

\(=\dfrac{19}{48}\)

c) \(\dfrac{-15}{25}-\dfrac{14}{21}+\dfrac{8}{5}-\dfrac{5}{3}\)

\(=\dfrac{-3}{5}-\dfrac{2}{3}+\dfrac{8}{5}-\dfrac{5}{3}\)

\(=\left(\dfrac{-3}{5}+\dfrac{8}{5}\right)-\left(\dfrac{2}{3}+\dfrac{5}{3}\right)\)

\(=\dfrac{5}{5}-\dfrac{7}{3}\)

\(=1-\dfrac{7}{3}\)

\(=-\dfrac{4}{3}\)

a: =-5/9+2/9-11/15-4/15

=-3/9-1

=-12/9=-4/3

b: =-8/13+21/13-7/16-1/6

=1-1/6-7/16

=5/6-7/16

=19/48

c: =-15/25+8/5-2/3-5/3

=-7/3+1

=-4/3

Ta có: \(S=\frac{1}{5.6}+\frac{2}{6.8}+\frac{3}{8.11}+\frac{4}{11.15}+\frac{5}{15.20}\)

\(\Rightarrow S=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+....+\frac{1}{15}-\frac{1}{20}\)

\(=\frac{1}{5}-\frac{1}{20}=\frac{4}{20}-\frac{1}{20}=\frac{3}{20}\)