Question

Tính :

S=\(\frac{4}{2.5}+\frac{4}{5.8}+\frac{4}{8.11}+...+\frac{4}{65.68}\)

Answer 1

S = 4/2.5 + 4/5.8 + 4/8.11 + ... + 4/65.48

S = 4/3 . ( 3/2.5 + 3/5.8 + 3/8.11 + ... + 3/65.68 )

S = 4/3 . ( 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/65 - 1/68 )

S = 4/3 . ( 1/2 - 1/68 )

S = 4/3 . 33/68

S = 11/17

Answer 2

\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)

Answer 3

\(=\frac{11}{17}\)

Answer 4

S=4/3.(3/2.5+3/5.8+3/8.11+........+3/65.68)

=4/3.(1/2-1/5+1/5-1/8+1/8-1/11+.........+1/65-1/68)

=4/3.(1/2-1/68)

=4/3.33/68

=11/17

Answer 5

Trả lời :

\(\frac{11}{17}\)

Hok tốt !

Answer 6

\(S=\frac{4}{2\cdot5}+\frac{4}{5\cdot8}+\frac{4}{8\cdot11}+....+\frac{4}{65\cdot68}\)

\(\frac{3}{4}S=\frac{3}{4}\left(\frac{4}{2\cdot5}+\frac{4}{5\cdot8}+\frac{4}{8\cdot11}+....+\frac{4}{65\cdot68}\right)\)

\(\frac{3}{4}S=\frac{3}{2\cdot5}+\frac{3}{5\cdot8}+\frac{3}{8\cdot11}+....+\frac{3}{65\cdot68}\)

\(\frac{3}{4}S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{65}-\frac{1}{68}\)

\(\frac{3}{4}S=\frac{1}{2}-\frac{1}{68}\)

\(\frac{3}{4}S=\frac{34}{68}-\frac{1}{68}=\frac{33}{68}\)

\(S=\frac{11}{17}\)