\(3C=\frac{3}{1.2.3.4}+\frac{3}{2.3.4.5}+...+\frac{3}{27.28.29.30}\)

\(3C=\frac{4-1}{1.2.3.4}+\frac{5-2}{2.3.4.5}+...+\frac{30-27}{27.28.29.30}\)

\(3C=\frac{1}{1.2.3}-\frac{1}{2.3.4}+\frac{1}{2.3.4}-\frac{1}{3.4.5}+...+\frac{1}{27.28.29}+\frac{1}{28.29.30}\)

\(3C=\frac{1}{1.2.3}-\frac{1}{28.29.30}\Rightarrow C=\left(\frac{1}{1.2.3}-\frac{1}{28.29.30}\right):3\)

Tính bằng cách thuận tiện nhất :

\(3,6\cdot4\cdot0,125\cdot2,5\cdot8\)

\(=3,6\cdot\left(4\cdot2,5\right)\cdot\left(0,125\cdot8\right)\)

\(=3,6\cdot10\cdot1\)

\(=36\)

a) 2,5 x 7,8 x 4                              b) 0,5 x 2 x 9,6

=(2,5 x 4) x 7,8                              =(0,5 x 2) x 9,6

=10          x 7,8                              =1 x 9,6

=78                                                =9,6

c)8,36 x 5 x 0,2                              d)8,3 x 7,9 + 7,9 x 1,7

=8,36 x (5 x 0,2)                             =7,9 + (8,3 x 1,7)

=8,36 x 1                                        =7,9 + 14,11

=8,36                                              =21,01

mong mn giúp đỡ

a: =43,8x(2,25+7,75)=43,8x10=438

b: =>12,25(121,3-a)=980

=>121,3-a=80

hay a=41,3

\(a,43.8.\left(2,25+7,75\right)=43,8.10=438\)

= (27/12 + 25/36)+(17/6 + 15/6)= 81/36 + 25/36 + 17/6 + 15/6 = 106/36 + 32/6 = 192/36 + 106/36 = 298/36 = 149/18

Tính thuận tiện :

\(\frac{12}{25}+\frac{3}{5}+\frac{13}{25}\)

= \(\frac{12}{25}+\frac{13}{25}+\frac{3}{5}\)

= \(\frac{25}{25}+\frac{3}{5}\)

= \(1+\frac{3}{5}\)

= \(\frac{5}{5}+\frac{3}{5}\)

= \(\frac{8}{5}\)

\(\frac{12}{25}+\frac{3}{5}+\frac{13}{25}\)

\(=\left(\frac{12}{25}+\frac{13}{25}\right)+\frac{3}{5}\)

\(=\frac{25}{25}+\frac{3}{5}\)

\(=1+\frac{3}{5}=\frac{8}{5}\)

\(\frac{12}{25}+\frac{3}{5}+\frac{13}{25}\)

\(=\left(\frac{12}{25}+\frac{13}{25}\right)+\frac{3}{5}\)

\(=\frac{25}{25}+\frac{3}{5}\)

\(=1+\frac{3}{5}\)

\(=\frac{8}{5}\)

a) 46 : 12 + 8 : 12

= (46 + 8) : 12

= 54 : 12

= 9/2

b) 17,89 : 25 - 4,89 : 25

= (17,89 - 4,89) : 25

= 13/25

c) 25,4 : 2,5 : 4

= (25,4 : 2,5) : 4

= 10 : 4

= 5/2

d) 2 : 28 × 7

= (2 × 7) : 28

= 14 : 28

= 1/2

c) 25

bang 8/5 nhe

=(12/25+13/25)+3/5

=1+3/5

=5/5+3/5

=8/5