Ta có : \(S=\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+\frac{1}{11.14}\)
\(\Rightarrow3S=\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+\frac{3}{11.14}\)
\(\Rightarrow3S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}\)
\(\Rightarrow3S=\frac{1}{2}-\frac{1}{14}=\frac{3}{7}\)
\(\Rightarrow S=\frac{3}{7}.\frac{1}{3}=\frac{1}{7}\)
3S= 1/2 - 1/5 + 1/5 - 1/8 + ... + 1/11 - 1/14
3S= 1/2 - 1/14
S= 3/7 / 3
S= 1/7
S=1/3 x (1/2 -1/5+1/5-......+1/11-1/14)
S=1/3 x (1/2-1/14)
S=1/3 x 3/7
S=1/7
S=1/2 x 5 + 1/5 x 8 + 1/8 x 11 + 1/11 x 14 S2=5/2 + 8/5 + 11/8 + 14/11 S3=2969/440 Bài này chỉ cần tính bằng máy tính là xong ^-^ nhớ mình nha. Xin cảm ơn!
