\(A=7+7^2+7^3+7^4+7^5+7^6+7^7+7^8\)

\(A=\left(7+7^3\right)+\left(7^2+7^4\right)+\left(7^5+7^7\right)+\left(7^6+7^8\right)\)

\(A=7\cdot\left(7+7^2\right)+7^2\cdot\left(1+7^2\right)+7^5\cdot\left(1+7^2\right)+7^6\cdot\left(1+7^2\right)\)

\(A=7\cdot50+7^2\cdot50+7^5\cdot50+7^6\cdot50\)

\(A=50\cdot\left(7+7^2+7^5+7^6\right)\)

\(A=5\cdot10\cdot\left(7+7^2+7^5+7^6\right)\)

Ta có: 5 ⋮ 5

⇒ \(A=5\cdot10\cdot\left(7+7^2+7^5+7^6\right)\) ⋮ 5 (đpcm) 

A = 7 + 72 + 73 + 74 + 75 + 76 + 77 + 78

A =  (7 + 73) + (72+ 74) + (75 + 77) + (76 + 78)

A = 7.(1 + 72)  + 72.(1 + 72) + 75.(1 + 72) + 76.(1 + 72)

A = 7.( 1 + 49) + 72.( 1 + 49) + 75.(1 + 49) + 76. (1 + 49)

A = 7.50 + 72.50 + 75.40 + 76.50

A = 50.(7 + 72 + 75 + 76)

Vì 50 ⋮ 5 nên A = 50.(7 + 72 + 76) ⋮ 5 đpcm

A = 7 + 7² + 7³ + 7⁴ + 7⁵ + 7⁶ + 7⁷ + 7⁸

A =  (7 + 7³) + (7²+ 7⁴) + (7⁵ + 7⁷) + (7⁶ + 7⁸)

A = 7.(1 + 7²)  + 7².(1 + 7²) + 7⁵.(1 + 7²) + 7⁶.(1 + 7²)

A = 7.( 1 + 49) + 7².( 1 + 49) + 7⁵.(1 + 49) + 7⁶. (1 + 49)

A = 7.50 + 7².50 + 7⁵.50 + 7⁶.50

A = 50.(7 + 7² + 7⁵ + 7⁶)

Vì 50 ⋮ 5 nên A = 50.(7 + 7² + 7⁶) ⋮ 5 đpcm

\(\Rightarrow x\times\left(1,8+\dfrac{1}{5}\right)=2,7\)

\(\Rightarrow x\times2=2,7\)

\(\Rightarrow x=\dfrac{2,7}{2}\)

\(\dfrac{-10}{25}=\dfrac{-2}{5}\)

-1/25-9/25=-10/25=-2/5

\(=\dfrac{\left(-1\right)-9}{25}=\dfrac{\left(-1\right)+\left(-9\right)}{25}=\dfrac{-10}{25}=\dfrac{-2}{5}\)

\(=\dfrac{32}{15}\cdot\dfrac{9}{17}\cdot\dfrac{3}{32}\cdot\dfrac{-17}{3}=-\dfrac{9}{3}=-3\)

=(22/15.3/32)   .   (9/17  :  -3/17)

=66/480     .     (-3)

=11/80  . (-3)

=-33/80

\(\left(2\dfrac{2}{15}\cdot\dfrac{9}{17}\cdot\dfrac{3}{32}\right):\left(-\dfrac{3}{17}\right)\)

\(=\left(\dfrac{32}{15}\cdot\dfrac{9}{17}\cdot\dfrac{3}{32}\right):\left(-\dfrac{3}{17}\right)\)

\(=\dfrac{9}{85}:\left(-\dfrac{3}{17}\right)\)

\(=\dfrac{9}{85}\cdot-\dfrac{17}{3}\)

\(=-\dfrac{3}{5}\)

Đề không rõ ràng. Bạn xem lại.

|-x - 5 | + 2 = 3

<=> | -x - 5| = 1

\(\Leftrightarrow\orbr{\begin{cases}-x-5=1\\-x-5=-1\end{cases}\Leftrightarrow\orbr{\begin{cases}x=-6\\-4\end{cases}}}\)

Tìm x biết : 

45 - [ ( 72 - 8 . x ) : 4 + 7 ] . 3  = 0 

[ ( 72 - 8 . x ) :4 + 7 ] . 3 = 45 - 0 

[ ( 72 - 8 . x) : 4 + 7 ] . 3 = 45 

( 72 - 8 . x ) : 4 + 7 = 45 : 3 

( 72 - 8 . x ) : 4 + 7 = 15

( 72 - 8 . x ) : 4 = 15 - 7 

( 72 - 8 . x ) : 4 = 8

72 - 8 . x  = 8 × 4 

72 - 8 . x = 32 

8 . x = 72 - 32 

8 . x = 40 

x = 40 : 8 

x = 5 

Vậy x = 5

\([\left(72-8\cdot x\right):4+7]\cdot3=45-0\)

\([\left(72-8\cdot x\right):4+7]\cdot3=45\)

\(\left(72-8\cdot x\right):4+7=45:3\)

\(\left(72-8\cdot x\right):4+7=15\)

\(\left(72-8\cdot x\right):4=15-7\)

\(\left(72-8\cdot x\right):4=8\)

\(72-8\cdot x=8\cdot4\)

\(72-8\cdot x=32\)

\(8x=72-32\)

\(8x=40\)

\(x=40:8\)

\(x=5\)