a) 731 – 680 + 19 = 51 + 19

                              = 70

b) 63 x 2 : 7 = 126 : 7

                    = 18

c) 14 x 6 – 29 = 84 – 29

                       = 55

d) 348 + 84 : 6 = 348 + 14

                        = 362

`@`Thay `x=2` vào `A` có:

 `A=3^2-9.2=9-18=-9`

`@` Thay `x=1/3` vào `A` có:

`A=(1/3)^2-9. 1/3=1/9-3=-26/9`

Khi x=2 thì \(A=3\cdot2^2-9\cdot2=12-18=-6\)

Khi x=1/3 thì \(A=3\cdot\dfrac{1}{9}-9\cdot\dfrac{1}{3}=\dfrac{1}{3}-3=-\dfrac{8}{3}\)

Bài 1: 

\(A=\dfrac{-1}{3}+1+\dfrac{1}{3}=1\)

\(B=\dfrac{2}{15}+\dfrac{5}{9}-\dfrac{6}{9}=\dfrac{2}{15}-\dfrac{1}{9}=\dfrac{18-15}{135}=\dfrac{3}{135}=\dfrac{1}{45}\)

\(C=\dfrac{-1}{5}+\dfrac{1}{4}-\dfrac{3}{4}=\dfrac{-1}{5}-\dfrac{1}{2}=\dfrac{-7}{10}\)

Bài 2: 

a: \(=\dfrac{1}{5}+\dfrac{1}{2}+\dfrac{2}{5}-\dfrac{3}{5}+\dfrac{2}{21}-\dfrac{10}{21}+\dfrac{3}{20}\)

\(=\left(\dfrac{1}{5}+\dfrac{2}{5}-\dfrac{3}{5}\right)+\left(\dfrac{2}{21}-\dfrac{10}{21}\right)+\left(\dfrac{1}{2}+\dfrac{3}{20}\right)\)

\(=\dfrac{-8}{21}+\dfrac{13}{20}=\dfrac{113}{420}\)

b: \(B=\dfrac{21}{23}-\dfrac{21}{23}+\dfrac{125}{93}-\dfrac{125}{143}=\dfrac{6250}{13299}\)

Bài 3:

\(\dfrac{7}{3}-\dfrac{1}{2}-\left(-\dfrac{3}{70}\right)=\dfrac{7}{3}-\dfrac{1}{2}+\dfrac{3}{70}=\dfrac{490}{210}-\dfrac{105}{210}+\dfrac{9}{210}=\dfrac{394}{210}=\dfrac{197}{105}\)

\(\dfrac{5}{12}-\dfrac{3}{-16}+\dfrac{3}{4}=\dfrac{5}{12}+\dfrac{3}{16}+\dfrac{3}{4}=\dfrac{20}{48}+\dfrac{9}{48}+\dfrac{36}{48}=\dfrac{65}{48}\)

Bài 4:

 \(\dfrac{3}{4}-x=1\)

\(\Rightarrow-x=1-\dfrac{3}{4}\)

\(\Rightarrow x=-\dfrac{1}{4}\)

Vậy: \(x=-\dfrac{1}{4}\)

\(x+4=\dfrac{1}{5}\)

\(\Rightarrow x=\dfrac{1}{5}-4\)

\(\Rightarrow x=-\dfrac{19}{5}\)

Vậy: \(x=-\dfrac{19}{5}\)

\(x-\dfrac{1}{5}=2\)

\(\Rightarrow x=2+\dfrac{1}{5}\)

\(\Rightarrow x=\dfrac{11}{5}\)

Vậy: \(x=\dfrac{11}{5}\)

\(x+\dfrac{5}{3}=\dfrac{1}{81}\)

\(\Rightarrow x=\dfrac{1}{81}-\dfrac{5}{3}\)

\(\Rightarrow x=-\dfrac{134}{81}\)

Vậy: \(x=-\dfrac{134}{81}\)

Lời giải:

$x=\frac{\sqrt{5}-1}{2}$

$2x=\sqrt{5}-1$

$2x+1=\sqrt{5}\Rightarrow (2x+1)^2=5$

$\Leftrightarrow 4x^2+4x-4=0$

$\Leftrightarrow x^2+x-1=0$

Khi đó:
\((4x^5+4x^4-5x^3+2x-2)^2\)

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

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

\(=(-1)^2=1\)

a. 1/3 + 1/4 - 1/6

= 7/12 - 1/6

= 5/12

b. 2/5 x 5/7 : 3/4

= 2/7 : 3/4

= 8/21

Gọi A là biểu thức trên

A=2x(1/1x2+1/2x3+....+1/19.20)

A=2x(1-1/2+1/2-1/3+1/3-1/4+....+1/19-1/20)

A=2x(1-1/20)

A=2x19/20

A=19/10

Vậy giá trị biểu thức trên là 19/10

Chúc em học tốt^^

a) \(=\dfrac{157}{8}.\dfrac{12}{7}-\dfrac{61}{4}.\dfrac{12}{7}=\dfrac{12}{7}\left(\dfrac{157}{8}-\dfrac{61}{4}\right)=\dfrac{12}{7}.\dfrac{35}{8}=\dfrac{15}{2}\)

b) \(\dfrac{2}{5}.\dfrac{1}{3}-\dfrac{2}{15}\div\dfrac{1}{5}+\dfrac{3}{5}.\dfrac{1}{3}=\dfrac{1}{3}\left(\dfrac{2}{5}+\dfrac{3}{5}\right)-\dfrac{2}{15}.5=\dfrac{1}{3}.1-\dfrac{2}{3}=\dfrac{1}{3}-\dfrac{2}{3}=-\dfrac{1}{3}\)

c) \(=-\dfrac{80}{9}\)

d) \(=-\dfrac{1}{6}\)

a.=\(\dfrac{157}{8}:\dfrac{7}{12}-\dfrac{61}{4}:\dfrac{7}{12}=\dfrac{471}{14}-\dfrac{183}{7}=\dfrac{15}{2}\)

b.=\(\dfrac{2}{15}-\dfrac{2}{3}+\dfrac{1}{5}=-\dfrac{1}{3}\)

c.\(\left(\dfrac{10}{3}+2.5\right):\left(\dfrac{19}{6}-\dfrac{21}{5}\right)-\dfrac{11}{31}=\dfrac{35}{6}:\left(-\dfrac{31}{30}\right)-\dfrac{11}{31}=-\dfrac{175}{31}-\dfrac{11}{31}=-6\)

d.\(\left[6+\dfrac{1}{8}-\dfrac{1}{2}\right]:\dfrac{3}{12}=\dfrac{45}{8}:\dfrac{3}{12}=\dfrac{45}{2}\)

Ta có : A = 1.2 + 2.3 + 3.4 + ...... + 19.20

=> 3A = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ...... + 19.20.21

=> 3A = 19.20.21

=> A = 19.20.21 / 3

=> A = 2660

1660 bạn nha

CHÚC BẠN HỌC GIỎI

a: \(A=x^2-2xy+y^2+x^2+2xy+y^2-2x^2-x\)

=-x

=-2