Hướng dẫn giải:

a) 3 x (20 – 5)

Cách 1:

3 x (20 – 5) = 3 x 15 = 45

Cách 2:

3 x (20 – 5) = 3 x 20 – 3 x 5 = 60 – 15 = 45

b) 20 x (40 – 1)

Cách 1:

20 x (40 – 1) = 20 x 39 = 780

Cách 2:

20 x (40 – 1) = 20 x 40 – 20 x 1 = 800 – 20 = 780

`1/2+2/4+3/6+4/8+5/10+6/12`

`=1/2+1/2+1/2+1/2+1/2+1/2`

`=1/2*6=3`

`1/3+1/4+1/5+8/10+20/15+20/30`

`=(1/3+1/4)+(1/5+4/5)+(4/3+2/3)`

`=7/12+1+2`

`=7/12+3=43/12`

\(\dfrac{1}{2}+\dfrac{2}{4}+\dfrac{3}{6}+\dfrac{4}{8}+\dfrac{5}{10}+\dfrac{6}{12}\)
\(=\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}\)
\(=\dfrac{1}{2}\times6=3\)
\(------\)
\(\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{8}{10}+\dfrac{20}{15}+\dfrac{20}{30}\)
\(=\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{4}{5}+\dfrac{4}{3}+\dfrac{2}{3}\)
\(=\left(\dfrac{1}{3}+\dfrac{4}{3}+\dfrac{2}{3}\right)+\left(\dfrac{1}{5}+\dfrac{4}{5}\right)+\dfrac{1}{4}\)
\(=\dfrac{7}{3}+1+\dfrac{1}{4}\)
\(=\dfrac{28}{12}+\dfrac{12}{12}+\dfrac{3}{12}\)
\(=\dfrac{43}{12}\)

`=1/2+1/2+1/2+1/2+1/2+1/2= 1/2 \times 6=3`

`----`

`=1/3+1/4+1/5+4/5+4/3+2/3 =(4/3+2/3+)+(1/5+4/5)+1/3+1/4=2+1+1/3+1/4=3+1/3+1/4=43/12`

 \(\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{8}{10}+\frac{15}{20}+\frac{20}{30}\)

= \(\left(\frac{1}{3}+\frac{20}{30}\right)+\left(\frac{1}{4}+\frac{15}{20}\right)\)\(+\left(\frac{1}{5}+\frac{8}{10}\right)\)

=     \(1+1+1\)

=   \(3\)

Hok tốt !!!

=(1/3+20/30)+(1/4+15/20)+(1/5+8/10)

=1+1+1

=2+1

=3

(1/3+ 20/30) + ( 1/4 + 15/20 ) + ( 1/5 + 8/10 )

= 1 + 1 + 1

= 3

ck bạn học tốt, tk mk nhé!

=20/60+15/60+12/60+48/60+40/60

=135/60

=9/4

\(\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{8}{10}+\dfrac{20}{30}+\dfrac{15}{20}\)

\(=\dfrac{20}{60}+\dfrac{15}{60}+\dfrac{12}{60}+\dfrac{48}{60}+\dfrac{40}{60}+\dfrac{45}{60}\)

\(=\dfrac{180}{60}\)

\(=3\)

mình bổ xung \(\dfrac{15}{20}\) rồi nhé

Đặt biểu thức là A, ta có:

\(A=\frac{x^{40}+x^{30}+x^{20}+x^{10}+1}{x^{45}+x^{40}+x^{35}+...+x^{10}+x^5+1}\)

\(\Rightarrow A.x^5=\frac{x^{45}+x^{35}+x^{25}+x^{15}+x^5}{x^{45}+x^{40}+x^{35}+...+x^{10}+x^5+1}\)

\(\Rightarrow A.x^5+A=\frac{x^{45}+x^{40}+x^{35}+x^{25}+x^{15}+x^5+x^{40}+x^{30}+x^{20}+x^{10}+1}{x^{45}+x^{40}+x^{35}+...+x^{10}+x^5+1}\)

\(\Rightarrow A.x^5+1=1\)

\(\Rightarrow A=\frac{1}{x^5+1}\)

a)\(\frac{X}{5}=\frac{5}{6}+\frac{-19}{30}\)

   \(\frac{X}{5}=\frac{1}{5}\)

Vậy \(X=1\)

b)\(X-\frac{41}{5}=\frac{-2}{3}\)

                   \(X=\frac{-2}{3}+\frac{41}{5}\)

                   \(X=\frac{113}{15}\)

Vậy \(X=\frac{113}{15}\)

c)\(\frac{31}{5}-X=\frac{11}{3}+\frac{7}{10}\)

    \(\frac{31}{5}-X=\frac{131}{30}\)

                   \(X=\frac{31}{5}-\frac{131}{30}\)

                    \(X=\frac{11}{6}\)

Vậy \(X=\frac{11}{6}\)

d)\(\frac{9}{X}=\frac{2}{5}+\frac{-7}{20}\)

   \(\frac{9}{X}=\frac{1}{20}\)

     \(X=9:\frac{1}{20}\)

     \(X=180\)

Vậy \(X=180\)

Hc tốt

a) 43/20 + 17/6 : ( 150/240 + 210/240 )

=    43/20 + 17/6 :  360/240

=    43/20 + 17/6 : 3/2

=    43/20 + 34/18

=     774/360 + 612/360

=   1386/360

=  77/20

b)

=  ( 5/30 + 3/30 + 2/30 ) : ( 5/30 + 3/30 - 2/30 )

=  10/30   :    0

=  1/3   :   0

=1/3

Đó là cách giải chi tiết va dễ hiểu nhất đó     ^ - ^