\(a.-\dfrac{1}{2}\)

\(b.3\)

\(c.5\)

Ta có 1/3x4= 1/3- 1/4

          2/4x6= 1/4-1/6

..............................

   5/13x18= 1/13 - 1/18

=> 1/3 -1/4 + 1/4 -1/6 +......+ 1/13 -1/18

  = 1/3 - 1/18

= 5/18

=1/3-1/4+1/4-1/6+1/6-1/9+...+1/13-1/18

=1/3-1/18

=5/18

k nha

=1/3-1/4+1/4-1/6+1/6-1/9+...+1/13-1/18

=1/3-1/18

=5/18

*** tk mk tk lk***

a)18:3 và (18.4):(3.4)

=6 và 72:12

=6 và 6

=> 6=6

b)81:9 và (81:3):(9:3)

=9 và 27:3

=9 và 9

=> 9=9

tick cho mk nha bạn

Ta có:\(\frac{1}{3x4}+\frac{2}{4x6}+\frac{3}{6x9}+\frac{4}{9x13}+\frac{5}{13x18}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}\)

\(=\frac{1}{3}-\frac{1}{18}=\frac{6}{18}-\frac{1}{18}=\frac{5}{18}\)

đặt \(A=\frac{1}{3.4}+\frac{2}{4.6}+\frac{3}{6.9}+\frac{4}{9.13}+\frac{5}{13.18}\)=\(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}=\frac{1}{3}-\frac{1}{18}=\frac{5}{18}\)

=>A=5/18

vậy........

\(\frac{1}{3.4}+\frac{1}{4.6}+\frac{1}{6.9}+\frac{1}{9.13}+\frac{5}{13.18}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}\)

\(=\frac{1}{3}-\frac{1}{18}\)

\(=\frac{5}{18}\)

Ta có :A = \(\frac{5}{3.4}+\frac{5}{4.6}+\frac{5}{5.8}+...+\frac{5}{40.78}=\frac{5}{2.2.3}+\frac{5}{2.3.4}+\frac{5}{2.4.5}+...+\frac{5}{2.39.40}\)

\(=\frac{5}{2}\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{39.40}\right)=\frac{5}{2}\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{39}-\frac{1}{40}\right)\)

\(=\frac{5}{2}.\left(\frac{1}{2}-\frac{1}{40}\right)=\frac{5}{2}.\frac{19}{40}=\frac{19}{16}\)

a,12x10+18x10=10x(12+18)

                     =10x30

                     =300

b,6x15+4x15=(6+4)x15

                 =10x15

                   =150

a, 12.10+18.10

=10(12+18)

=10.30

=300

b,6.15+4.15

=15(6+4)

=15.10

=150

12 x 10 + 18 x 10

= (12 + 18 ) x 10

= 30 x 10

= 300.

b, 6 x 15 x 4 x 15

= (6 x 4 )  x 15

= 24 x 15

= 360.

82+18x10+90

= 82+(180+90)

= 82+270

=352

= 82 + 180 + 90

= 262 + 90

= 352

352 nha bạn1!

k cho mình nha ! thanks

Tham Khảo :

= 32 x 8 + 9 x 8+9 x 8

=64.4+18.4+18.4

=(64+18+18).4

=100.4

=400

=32×8+9×8+9×8=8(32+9+9)=8×50=400

Ta có: \(64.4+18.4+9.8\)

\(=64.4+18.4+9.2.4\)

\(=64.4+18.4+18.4\)

\(=4.\left(64+18+18\right)=4.100=400\)

Ủng hộ nha?

\(\frac{9\cdot6-9\cdot2}{18}\)

\(=\frac{9\left(6-2\right)}{18}\)

\(=\frac{9\cdot4}{18}\)

\(=\frac{36}{18}\)

\(=2\)

\(\frac{9.6-9.2}{18}\)

\(=\frac{9.\left(6-2\right)}{18}\)

\(=\frac{9.4}{18}\)

\(=\frac{36}{12}\Leftrightarrow2\)

Chúc bạn học tốt !

 9 x 6 - 9 x 2 / 18

= 9 x ( 6 - 2) / 18

= 9 x 4 / 18

=  9 x 4 / 9 x 2

= 4 /2 = 2 / 1 = 2