Question

[Phạm Quang Huy]

Tính :

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

Answer 1

[Nguyễn Đình Dũng]

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

[Huy Hoàng Đoàn Lê]

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