a,    2 \(\times\) \(\dfrac{2}{4}\) : \(\dfrac{4}{5}\)                                  b, \(\dfrac{7}{8}\) \(\times\) \(\dfrac{4}{5}\) : 3

=              1 :  \(\dfrac{4}{5}\)                              =      \(\dfrac{7}{10}\)       : 3

=              1 \(\times\) \(\dfrac{5}{4}\)                            =         \(\dfrac{7}{10}\)  \(\times\) \(\dfrac{1}{3}\)

=                 \(\dfrac{5}{4}\)                               =            \(\dfrac{7}{30}\)

\(2\times\dfrac{2}{4}:\dfrac{4}{5}\)

\(=1:\dfrac{4}{5}\)

\(=1\times\dfrac{5}{4}\)

\(=\dfrac{5}{4}\)

__________________________

\(\dfrac{7}{8}\times\dfrac{4}{5}:3\)

\(=\dfrac{7}{2}\times\dfrac{1}{5}:3\)

\(=\dfrac{7}{10}\times\dfrac{1}{3}\)

\(=\dfrac{7}{30}\)

 Chúc bạn học tốt:>

D

C

Giúp mình

= 3/4 + 1/3 = 13/12

5/4 x X = 5/8

X = 5/8 : 5/4

X = 1/2

vậy X = ...

\(\dfrac{3}{5}:\dfrac{4}{5}+\dfrac{1}{2}\times\dfrac{2}{3}=\dfrac{3}{4}+\dfrac{1}{3}+\dfrac{9}{12}+\dfrac{4}{12}=\dfrac{11}{12}\)

\(\dfrac{5}{4}\times x=\dfrac{5}{8}\)

\(x=\dfrac{5}{8}:\dfrac{5}{4}\)

\(x=\dfrac{1}{2}\)

Tính giá trị biểu thức
   \(\dfrac{3}{5}:\dfrac{4}{5}+\dfrac{1}{2}x\dfrac{2}{3}=\dfrac{3}{5}x\dfrac{5}{4}+\dfrac{1}{3}=\dfrac{3}{4}+\dfrac{1}{3}=\dfrac{9+4}{12}=\dfrac{13}{12}\)  

Tìm x
    \(\dfrac{5}{4}\)xX=\(\dfrac{3}{8}+\dfrac{1}{4}\)
    \(\dfrac{5}{4}\)xX=\(\dfrac{5}{8}\)
    x=\(\dfrac{5}{8}\) :\(\dfrac{5}{4}\)
    x=\(\dfrac{1}{2}\)

Bài 1

1/2 x 3/4 : 4/5 

= 3/8 : 4/5 

= 3/8 x 5/4

= 15/32

2/3 + 1/6 - 1/2

= 4/6 + 1/6 - 3/6

= 5/6 - 3/6

= 1/3

3/5 + 4 : 2/3 - 3/2

= 3/5 + 4 x 3/2 - 3/2

= 3/5 + 6 - 3/2

= 3/5 + 30/5 - 3/2

= 33/5 - 3/2

= 66/10 - 15/10

= 28/5

Bài 2

x - 1/4 = 1/2 

x         = 1/2 + 1/4

x         = 2/4 + 1/4 

x         = 3/4

2/3 + x = 1

         x = 1 - 2/3 

         x = 3/3 - 2/3

         x = 1/3

1/2 x 3/4 : 4/5

=1/2 x 3/4 x 4/5

=3/8 x 4/5

=3/10

(-25).(-3).(-4)=-300

(-1).(-4).5.8.25=4000

C, (2ab mũ 2) chia C Với a=4;b=-6;C=12

(2ab^2):c với a=4;b=-6;c=12

(2ab^2):c=(2.4.-6):12

                  =(-48):12

                  = - 4    

E, ( a mũ 2 – b mũ 2 ) : (a+b) (a–b) với a=5, b= -3

(a^2-b^2):(a+b).(a-b) với a=5;b=-3

(a^2 - b^2):(a+b).(a-b) = (5^2 - (-3)^2):(5+(-3)).(5 - (-3)

                                    = 64

(-25).(-3).(-4)=-300

a,

\(A=4(x-2)(x+1)+(2x-4)^2+(x+1)^2\\=[2(x-2)]^2+2\cdot2(x-2)(x+1)+(x+1)^2\\=[2(x-2)+(x+1)]^2\\=(2x-4+x+1)^2\\=(3x-3)^2\)

Thay $x=\dfrac12$ vào $A$, ta được:

\(A=\Bigg(3\cdot\dfrac12-3\Bigg)^2=\Bigg(\dfrac{-3}{2}\Bigg)^2=\dfrac94\)

Vậy $A=\dfrac94$ khi $x=\dfrac12$.

b,

\(B=x^9-x^7-x^6-x^5+x^4+x^3+x^2-1\\=(x^9-1)-(x^7-x^4)-(x^6-x^3)-(x^5-x^2)\\=[(x^3)^3-1]-x^4(x^3-1)-x^3(x^3-1)-x^2(x^3-1)\\=(x^3-1)(x^6+x^3+1)-x^4(x^3-1)-x^3(x^3-1)-x^2(x^3-1)\\=(x^3-1)(x^6+x^3+1-x^4-x^3-x^2)\\=(x^3-1)(x^6-x^4-x^2+1)\)

Thay $x=1$ vào $B$, ta được:

\(B=(1^3-1)(1^6-1^4-1^2+1)=0\)

Vậy $B=0$ khi $x=1$.

$Toru$

\(4^3:8\cdot3^2-5^2+9\)

\(=8\cdot9-25+9\)

=81-25

=56