a)=1x2x3x4x5/2x3x4x5x6 =1/6

b)=1x2x3x4x5x6x7x8x9/10x9x8x7x6x5x4x3 x2=1/10

6x1=6

6x2=12

6x3=18

6x4=24

6x5=30

6x6=36

6x7=42

6x8=48

6x9=54

6x10=60

đúng thig k nhé

6 x 1 = 6

6 x 2 = 12

6 x 3 = 18

6 x 4 = 24

6 x 5 = 30

6 x 6 = 36

6 x 7 = 42

6 x 8 = 48

6 x 9 = 54

6 x 10 = 60

6x1=6

6x2=12

6x3=18

6x4=24

6x5=30

6x6=36

6x7=42

6x8=48

6x9=54

6x10=60

\(9\times8+123\)

\(=72+123\)

\(=195\)

\(vay:72+123=195nha\)

9x8+123=195

9*8+123

=72+123

=195

6*7=42

6 x 7 = 42

42 bạn ạ k cho mình nha

Lời giải:
$2\times A=\frac{2}{1\times 3}+\frac{2}{3\times 5}+\frac{2}{5\times 7}+...+\frac{2}{19\times 21}$
$2\times A=\frac{3-1}{1\times 3}+\frac{5-3}{3\times 5}+\frac{7-5}{5\times 7}+...+\frac{21-19}{19\times 21}$

$=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+....+\frac{1}{19}-\frac{1}{21}$

$=1-\frac{1}{21}=\frac{20}{21}$

$\Rightarrow A=\frac{20}{21}: 2= \frac{10}{21}$

\(a)\) \(4.126.25=\left(4.25\right).126=100.126=12600\)

a,4x126x25

=(4x25)x126

=100x126

=12600

cảm ơn 2 bn nhé thank you 

9x2x4=72

9x8+4=76

4x2x9-2=70

~HT~ {học tốt}

TL : 

9 x 2 x 4 = 18 x 4 = 72

9 x 8 + 4 = 72 + 4 = 76

4 x 2 x 9 - 2 = 8 x 9 - 2 = 72 - 2 = 70

T I C K cho mình nhé !

~HT~

= 72

= 76

= 70

Hok tốt

mình cần gấp

\(\frac{2}{5\times6}+\frac{2}{6\times7}+...+\frac{2}{68\times69}\)

\(=\left(\frac{1}{5\times6}+\frac{1}{6\times7}+...+\frac{1}{68\times69}\right)\times2\times\frac{1}{2}\)

\(=\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{68}-\frac{1}{69}\right)\times1\)

\(=\left(\frac{1}{5}-\frac{1}{69}\right)\times1\)

\(=\frac{64}{345}\times1=\frac{64}{345}\)

#Nii

1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/6x7

=1/1-1/2+1/2-1/3+...-1/7

=1+(1/2-1/2+1/3-1/3+...+1/6-1/6)-1/7

=1 +0+0+...-1/7

=1-1/7

=6/7

hình như là 8/7
 

Kếết quả là 116/24

Rút gọn ra phân số tối giản là 29/6