\(331.997+662+331\)                              (* Dấu " . " là dấu nhân )

 \(=331.997+331.2+331.1\)

\(=331.\left(997+2+1\right)\)

\(=331.1000\)

\(=331000\)

331*997 + 622 + 331

= 331*997 + 331*2 + 331*1

= 331*(997 + 2 + 1)

= 331*1000

= 331000

( 0,2 + 0,4 + 0,6 + ... + 3,2 ) x ( 4,5 x 3,6 - 8,1 x 2 )

= ( 0,2 + 0,4 + 0,6 + ... + 3,2 ) x ( 16,2 - 16,2 )

= ( 0,2 + 0,4 + 0,6 + ... + 3,2 ) x 0

= 0

\(\frac{x}{4}=\frac{9}{x}\)

\(\Leftrightarrow x.x=9.4\)

\(\Leftrightarrow x^2=36\)

\(\Leftrightarrow x^2=6^2\)

\(\Leftrightarrow x=6\)

x/4 = 9/x

=> 9.4=x.x

=> 36 = x²

=> 6²=x²

=> x = 6

câu 1:

78 . 45 + 44 . 22

=78 . 44 + 44 . 22 + 1 . 78

=44 . (78 + 22) + 78

=44 . 100 + 78

=4400 + 78

=4478

Lời giải:
$22+23-25+27-29+31-33$

$=22+(23-25)+(27-29)+(31-33)$

$=22+(-2)+(-2)+(-2)=22+(-2).3=22-6=16$

\(\dfrac{-1}{9}.\dfrac{-3}{5}+\dfrac{5}{-6}.\dfrac{-3}{5}-\dfrac{7}{2}.\dfrac{3}{5}\)

\(=\dfrac{3}{5}.\left(\dfrac{1}{9}+\dfrac{5}{6}-\dfrac{7}{2}\right)\)

\(=\dfrac{3}{5}.\left(\dfrac{2}{18}+\dfrac{15}{18}-\dfrac{63}{18}\right)\)

\(=\dfrac{3}{5}.\left(-\dfrac{23}{9}\right)\)

\(=-\dfrac{69}{45}\)

a: \(=\dfrac{-39+19+10}{12}=\dfrac{-10}{12}=\dfrac{-5}{6}\)

b: \(=\dfrac{2^{30}\cdot3^{16}\cdot7-2^{34}\cdot3^{15}}{2^{28}\cdot3^{21}-2^{28}\cdot3^{17}}\)

\(=\dfrac{2^{30}\cdot3^{15}\left(3\cdot7-2^4\right)}{2^{28}\cdot3^{17}\left(3^4-1\right)}=\dfrac{2^2}{3^2}\cdot\dfrac{21-16}{80}=\dfrac{4}{9}\cdot\dfrac{5}{80}\)

\(=\dfrac{20}{720}=\dfrac{1}{36}\)

A= 1/3 + 1/3^2 + ... + 1/3^8

3A= 3. (1/3+ 1/3^2+ ... + 1/3^8)

3A=1+ 1/3 + 1/3^2+ ... +1/3^7

=> 3A - A= (1 + 1/3 + 1/3^2 + ... + 1/3^7) - (1/3 + 1/3^2+ ... + 1/3^8)

=> 2A= 1 - 1/ 3^8

2A= 6560/6561

A= 6560/6561 : 2

A= 3280/6561

nè bạn

`6/7 . 8/13 +6/7 . 9/13+3/13 . 6/7`

`=6/7 . (8/13+9/13+3/13)`

`=6/7 . 20/13`

`=120/91`

\(\dfrac{6}{7}.\dfrac{8}{13}+\dfrac{6}{7}.\dfrac{9}{13}+\dfrac{3}{13}.\dfrac{6}{7}\)

\(=\dfrac{6}{7}.\left(\dfrac{8}{13}+\dfrac{9}{13}+\dfrac{3}{13}\right)\)

\(=\dfrac{6}{7}.\left(\dfrac{8+9+3}{13}\right)\)

\(=\dfrac{6}{7}.\dfrac{20}{13}\)

\(=\dfrac{6.20}{7.13}\)

\(=\dfrac{120}{91}\)