BAI 1 ; 

Bài 2: 

a, \(\dfrac{5}{23}\) \(\times\) \(\dfrac{17}{26}\) + \(\dfrac{5}{23}\) \(\times\) \(\dfrac{9}{26}\) 

= \(\dfrac{5}{23}\) \(\times\) ( \(\dfrac{17}{26}\) + \(\dfrac{9}{26}\))

= \(\dfrac{5}{23}\) \(\times\) \(\dfrac{26}{26}\)

= \(\dfrac{5}{23}\) 

b, \(\dfrac{3}{4}\) \(\times\) \(\dfrac{7}{9}\) + \(\dfrac{7}{4}\)  \(\times\) \(\dfrac{3}{9}\)

= \(\dfrac{7}{12}\) + \(\dfrac{7}{12}\)

= \(\dfrac{14}{12}\)

= \(\dfrac{7}{6}\)

a)\(\frac{4}{3.7}+\frac{4}{7.11}+...+\frac{4}{23.27}=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{23}-\frac{1}{27}=\frac{1}{3}-\frac{1}{27}=\frac{8}{27}\)

b)\(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{6.7}=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{6}-\frac{1}{7}=\frac{1}{2}-\frac{1}{7}=\frac{5}{14}\)

c)\(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{11.13}+\frac{2}{1.2}+\frac{2}{2.3}+...+\frac{2}{9.10}=\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{11}-\frac{1}{13}\right)+2\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(=\frac{1}{3}-\frac{1}{13}+2\left(1-\frac{1}{10}\right)=\frac{10}{39}+\frac{9}{5}=\frac{401}{195}\)

a)\(2-3+5-7+9-11+13-15+17=\left(2+5+9+13+17\right)-\left(3+7+11+15\right)\)

                                                                                               \(=46-36=10\)

b)\(\frac{1}{1.2}+\frac{1}{2.3}+...............+\frac{1}{8.9}=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+.................+\frac{1}{8}-\frac{1}{9}\)

                                                                        \(=\frac{1}{1}-\frac{1}{9}=\frac{9}{9}-\frac{1}{9}=\frac{8}{9}\)

Áp dụng \(\frac{1}{n.\left(n+1\right)}=\frac{1}{n}-\frac{1}{n+1}\)

Chúc bạn học tốt

Đề là tính bằng cách hợp lý đúng ko bạn

a, 2-3+5-7+9-11+13-15+17

= (5+13) - (3+15) + (2+9-11) + (17-7)

= 18 - 18 + 0 +10

= 10

b, \(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}\)

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

\(=1-\frac{1}{9}\)

\(=\frac{8}{9}\)

Bài 2. > , < ,  = ?

5/7 < 4/3             2/5 < 6/10            1/4 = 3/12                27/36 >

2/9           7/6 > 7/9

7/2 = 2/7        15/23 < 1                  27/9 > 2          14/15 < 1          51/17 < 4

a) 1/2
b) 18/17; 17/16; 16/15; 15/14; 14/13; 13/12; 10/9
Nhớ k cho mk nha

a)1/2

b)Vì 18/17 <17/16 <16/15<15/14< 14/13< 13/12< 10/9

Nên, ta có: 

18/17; 17/16; 16/15; 15/14; 14/13; 13/12; 10/9.

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10=55

2 + 4 + 6 + 8 + 10 + 12 + 14 + 16 + 18 + 20=110

1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19=100

k mk nha pạn hiền!

10 +1 = 11 có 10 số vậy thì 10 : 2 = 5 thì sẽ bằng 5x11 = 55

2 + 20 = 22 cũng có 10 số lấy 10 : 2 = 5 se bang 22 x5 = 110

1+19 = 20 , 10 : 2 = 5 , 20x5 = 100

b)\(4\times\dfrac{3}{8}=\dfrac{12}{8}=\dfrac{3}{2}\)

    \(3:\dfrac{1}{2}\) 

   =\(3\times\dfrac{2}{1}\)

  =  6   

  =\(\dfrac{1}{2}:3\)

  = \(\dfrac{1}{2}\times\dfrac{1}{3}\)

  = \(\dfrac{1}{6}\)

1a, \(\dfrac{3}{10}\) \(\times\) \(\dfrac{4}{9}\) 

= \(\dfrac{3\times2\times2}{2\times5\times3\times3}\) 

= \(\dfrac{2}{15}\)

\(\dfrac{6}{5}\): \(\dfrac{3}{7}\)

= \(\dfrac{6}{5}\) \(\times\) \(\dfrac{7}{3}\)

= \(\dfrac{14}{5}\)

\(\dfrac{3}{4}\) \(\times\) \(\dfrac{2}{5}\)

= \(\dfrac{3\times2}{2\times2\times5}\)

= \(\dfrac{3}{10}\)

\(\dfrac{5}{8}\) : \(\dfrac{1}{2}\)

= \(\dfrac{5}{8}\) \(\times\) \(\dfrac{2}{1}\)

= \(\dfrac{5}{4}\)

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