Question

[Lê Minh Đức]

CMR : S = 1/5+1/13+1/14+1/15+1/61+1/62+1/63 < 1/2

minh thach ai lam duoc bai nay

Answer 1

[Đặng Tiến]

\(S=\frac{1}{5}+\left(\frac{1}{13}+\frac{1}{14}+\frac{1}{15}\right)+\left(\frac{1}{61}+\frac{1}{62}+\frac{1}{63}\right)\)

\(\Rightarrow S< \frac{1}{5}+\frac{1}{12}.3+\frac{1}{60}.3\)

\(\Rightarrow S< \frac{1}{5}+\frac{1}{4}+\frac{1}{20}\)

\(\Rightarrow S< \frac{1}{2}\)