\(B=\dfrac{1}{6\cdot10}+\dfrac{1}{10\cdot14}+...+\dfrac{1}{402\cdot406}\)

\(=\dfrac{1}{4}\left(\dfrac{1}{6}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{14}+...+\dfrac{1}{402}-\dfrac{1}{406}\right)\)

\(=\dfrac{1}{4}\cdot\dfrac{400}{2436}=\dfrac{100}{2436}=\dfrac{25}{609}\)

12:6x10=2 x 10 = 20

45+23x234 = 45 + 5382 = 5427

k mình nha!

A= 1/4 ( 1/2x6 +1/6x10 +.............+1/46x50)

A= 1/4 ( 1/2 - 1/6 + 1/6 - 1/10 +.......... + 1/46 - 1/50 )

A= 1/4 ( 1/2 - 1/50 )

A= 1/4 x 12/25

A= 3/25

3/25 nha

thanks nha

\(\frac{2}{1.3}+\frac{3}{3.6}+\frac{4}{6.10}+\frac{5}{10.15}+\frac{6}{15.21}\)

= \(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\)

= \(1-\frac{1}{21}\)

= \(\frac{20}{21}\)

Ủng hộ mk nha !!! ^_^

số đó là

90000:6x10=150000

100:25x6=24

1000:7x21=3000

đúng ko bn

90000 : 6 x 10 = 900000 : 6 

                        = 150000

100 : 25  x 6 = 4 x6

                      = 24

1000 : 7 x 21 = 21000 : 7

                        = 3000 

nhé !

a) 90000:6x10

=15000x10

=150000

b) 100:25x6

=4x6

=24

c) 1000:7x21

=1000:1x3

=1000x3

=3000

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

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

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