90

tick cho tròn 400 nha

vì  2.3=6

3.4=12

4.5=20

5.6=30

6.7=42

7,8=56

8.9=72

9.10=90

vậy =90

2x3=6, 3x4=12, 4x5=20, 5x6=30, 6x7=42, 9x10=90!!!

đó

2x3=6

3x4=12

4x5=20

....

9x10=90

=>9=90

1+1+2+2+3+3+4+4+5+5+6+6+7+7+8+9+8+9+10+12+13+14+15+16+17+18+19+20+30+1000000=10000274

10000274

9 = 90

Vì theo quy luật

6 = 2.3

12=3.4

20 = 4.5

...

90 = 9.10

Tick đúng nha!!!

Ở đây quy luật là cứ 1 đơn vị tăng là số nhân tăng theo

=> 2 = 6 có nghĩa 2 x 3 = 6

Vậy 9 = 9 x 8 = 72.

Vậy 9 = 72.

9 = 90 vì mỗi lần nhân thêm 1 đơn vị

a; \(\dfrac{1}{4}\) + \(\dfrac{2}{5}\) + \(\dfrac{6}{8}\) + \(\dfrac{9}{15}\) + \(\dfrac{8}{1}\)

= (\(\dfrac{1}{4}\) + \(\dfrac{6}{8}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{9}{15}\)) + \(\dfrac{8}{1}\)

= (\(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{3}{5}\)) + 8

=  1 + 1 + 8

=  2 + 8

= 10

b; \(\dfrac{1}{2}\) + \(\dfrac{2}{4}\) + \(\dfrac{3}{6}\) + \(\dfrac{4}{8}\) + \(\dfrac{5}{10}\) + \(\dfrac{6}{12}\) + \(\dfrac{7}{14}\) + \(\dfrac{8}{16}\) + \(\dfrac{10}{20}\)

=  \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x (\(\dfrac{2}{2}\) + \(\dfrac{3}{3}\) + \(\dfrac{4}{4}\) + \(\dfrac{5}{5}\)+ \(\dfrac{6}{6}+\dfrac{7}{7}+\dfrac{8}{8}\) + \(\dfrac{10}{10}\))

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

= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x 1 x 8

= \(\dfrac{1}{2}\) + \(\)\(\dfrac{1}{2}\) x 8

= \(\dfrac{1}{2}\) + 4

= \(\dfrac{9}{2}\) 

a; \(\dfrac{1}{4}\) + \(\dfrac{2}{5}\) + \(\dfrac{6}{8}\) + \(\dfrac{9}{15}\) + \(\dfrac{8}{1}\)

  = (\(\dfrac{1}{4}\) + \(\dfrac{6}{8}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{9}{15}\)) + 8

= (\(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{3}{5}\)) + 8

= 1 + 1 + 8

= 2 + 8

= 10

b; \(\dfrac{1}{2}\) + \(\dfrac{2}{4}\) + \(\dfrac{3}{6}\) + \(\dfrac{4}{8}\) + \(\dfrac{5}{10}\) + \(\dfrac{6}{12}\) + \(\dfrac{7}{14}\) + \(\dfrac{8}{16}\) + \(\dfrac{9}{18}\) + \(\dfrac{10}{20}\)

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

= \(\dfrac{1}{2}\) x 10

= 5

12 phút 30 giây * 6 = 72 phút 180 giây hay 1 giờ 15 phút

3 giờ 20 phút : 5 = 0 giờ 40 phút

\(\frac{5}{6}:\left(y-\frac{7}{9}\right)=\frac{3}{4}\)

\(\left(y-\frac{7}{9}\right)=\frac{5}{6}:\frac{3}{4}\)

\(y-\frac{7}{9}=\frac{10}{9}\)

\(y=\frac{10}{9}+\frac{7}{9}\)

\(y=\frac{17}{9}\)

74 ' 20 giây

40'

5/6 : ( y + 7/9 ) = 3/4

          y + 7/9   = 5/6 : 3/4

          y + 7/9   = 10/9

          y            = 10/9 - 7/9

          y            = 1/3

Mình làm trước nên **** !

Nếu:2=6

       3=12

       4=20

       5=30

       6=42

Thì: 9=90

k nha!

9 thì bằng 9 chứ bao nhiêu

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