28 + 21 : 7 = 28 + 3 = 31

(28 + 21) : 7 = 49 : 7 = 7

Học sinh nhẩm và ghi kết quả như sau:

35 : 5 = 7

28 : 7 = 4

24 : 6 = 4

21 : 3 = 7

35 : 7 = 5

28 : 4 = 7

24 : 4 = 6

21 : 7 = 3

35 : 5 = 7

28 : 7 = 4

24 : 6 = 4

21 : 3 = 7

35 : 7 = 5

28 : 4 = 7

24 : 4 = 6

21 : 7 = 3

28/35+21/28+7/21+1/7

=4/5+3/4+1/3+1/7

=851/420

dùng máy tính là ok mà bn.

\(\frac{28}{35}+\frac{21}{28}+\frac{9}{21}+\frac{1}{7}\)

\(=(\frac{4}{5}+\frac{3}{4})+(\frac{3}{7}+\frac{1}{7})\)

\(=\frac{31}{20}+\frac{4}{7}\)

\(=2\frac{17}{140}\)

Chúc bn học tốt!!!!

28 + 21 : 7 = 28 + 3 = 31

\(\dfrac{26}{81}+\dfrac{4}{27}=\dfrac{26+12}{81}=\dfrac{38}{81}\)

\(\dfrac{5}{64}+\dfrac{7}{8}=\dfrac{5+56}{64}=\dfrac{61}{64}\)

\(\dfrac{26}{81}+\dfrac{4}{27}=\dfrac{26}{81}+\dfrac{4\times3}{27\times3}=\dfrac{26}{81}+\dfrac{12}{81}=\dfrac{38}{81}\)

\(\dfrac{5}{64}+\dfrac{7}{8}=\dfrac{5}{64}+\dfrac{7\times8}{8\times8}=\dfrac{5}{64}+\dfrac{56}{64}=\dfrac{61}{64}\)

S=7(1+1/3+1/6+...+1/55)

=7(2/2+2/6+...+2/110)

=14(1/2+1/6+...+1/110)

=14(1-1/2+1/2-1/3+...+1/10-11)

=14*10/11=140/11

\(S=7+\dfrac{7}{3}+\dfrac{7}{6}+\dfrac{7}{10}+\dfrac{7}{15}+\dfrac{7}{21}+\dfrac{7}{28}+\dfrac{7}{36}+\dfrac{7}{45}+\dfrac{7}{55}\\ =7+\left(\dfrac{7}{3}+\dfrac{7}{21}\right)+\left(\dfrac{7}{6}+\dfrac{7}{36}\right)+\left(\dfrac{7}{15}+\dfrac{7}{45}\right)+\dfrac{7}{10}+\dfrac{7}{28}+\dfrac{7}{55}\\ =7+\dfrac{8}{3}+\dfrac{49}{36}+\dfrac{28}{45}+\dfrac{7}{10}+\dfrac{7}{28}+\dfrac{7}{55}\\ =7+\left(\dfrac{8}{3}+\dfrac{49}{36}+\dfrac{28}{45}\right)+\dfrac{7}{10}+\dfrac{7}{28}+\dfrac{7}{55}\\ =7+\dfrac{93}{20}+\dfrac{7}{10}+\dfrac{7}{28}+\dfrac{7}{55}\\ =7+\left(\dfrac{93}{20}+\dfrac{7}{10}\right)+\dfrac{7}{28}+\dfrac{7}{55}\\ =7+\dfrac{107}{20}+\dfrac{7}{28}+\dfrac{7}{55}\\ =\dfrac{140}{11}\)

hehe ko biết

Học sinh nhẩm và ghi kết quả như sau:

28 : 7 = 4

70 : 7 = 10

21 : 7 = 3

42 : 7 = 6

14 : 7 = 2

56 : 7 = 8

63 : 7 = 9

42 : 6 = 7

49 : 7 = 7

35 : 7 = 5

7 : 7 = 1

0 : 7 = 0

@4

@10

@3

28:7=4

70:7=10

21:7=3

42:7=6

14:7=2

56:7=8

63:7=9

42:6=7

49:7=7

35:7=5

7:7=1

0:7=0

    a, 24÷6 = 4

 b, 28÷7 = 4

 c, 42÷6 = 7

    d, 21÷3 = 7

 a) (9 + 6) x 4 =15x4=60

     9 + 6 x 4  =9+24=33

b)  28 + 21 : 7 = 28+3=31

     (28 + 21) : 7 = 49:7=7

a) (9 + 6) x 4 = 60

     9 + 6 x 4 = 33

b) 28 + 21 : 7 = 31

    (28 + 21) : 7 = 8,4285

 a) (9 + 6) x 4 = 15 x 4 = 60

      9 + 6 x 4  = 9 + 24 = 33              

b)  28 + 21 : 7 = 28 + 3 = 31

      (28 + 21) : 7 = 49 : 7 =7