7x1x7=49

7x2x7=98

7x3x7=147

7x4x7=196

7x5x7=245

7x6x7=294

7x7x7=343

7x8x7=392

7x9x7=441

7x10x7=490

7 x 1 = 7

7 x 2 = 14

7 x 3 = 21

7 x 4 = 28

7 x 5 = 35

7 x 6 = 42

7 x 7 = 49

7 x 8 = 56

7 x 9 = 63

7 x 10 = 70.

77 - 7 + 7 + 7 + ... + 7 = 70 + 70 = 140

7+7+.......+7+7 có 11 số hạng tức là bằng 7 x 11.

7 x 11 = 77.

77 - 77 = 0

\(\dfrac{2}{5}+\dfrac{3}{7}=\dfrac{14}{35}+\dfrac{15}{35}=\dfrac{29}{35}\)

\(\dfrac{1}{7}+\dfrac{3}{7}+\dfrac{4}{7}=\left(\dfrac{1}{7}+\dfrac{3}{7}\right)+\dfrac{4}{7}=\dfrac{1}{7}+\left(\dfrac{3}{7}+\dfrac{4}{7}\right)=\left(\dfrac{1}{7}+\dfrac{4}{7}\right)+\dfrac{3}{7}\)

=12/7

\(\frac{12}{7}\)nghe bn

#)Giải :

Đặt \(A=\frac{7}{2}+\frac{7}{4}+\frac{7}{8}+\frac{7}{16}+\frac{7}{32}+...+\frac{7}{512}\)

\(A=\frac{7}{2}+\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+...+\frac{7}{2^9}\)

\(\Rightarrow2A=\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+\frac{7}{2^5}+...+\frac{7}{2^{10}}\)

\(\Rightarrow2A-A=\left(\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+\frac{7}{2^5}+...+\frac{7}{2^{10}}\right)-\left(\frac{7}{2}+\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+...+\frac{7}{2^9}\right)\)

\(\Rightarrow A=\frac{7}{2^{10}}-\frac{7}{2}\)

tiếc nhỉ giải theo kiểu học sinh lớp 4 cơ

vo van , vien vong