Question

đây là hai bài về Tính nhanh nha!!!

 

Answer 1

\(a,S=\dfrac{3}{1.4}+\dfrac{3}{4.7}+\dfrac{3}{7.10}+\dfrac{3}{10.13}+\dfrac{3}{13.16}\)

\(\Rightarrow S=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{16}\)

\(\Rightarrow S=1-\dfrac{1}{16}\\ \Rightarrow S=\dfrac{15}{16}\)

\(b,S=\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{49.50}\)

\(\Rightarrow S=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{49}-\dfrac{1}{50}\)

\(\Rightarrow S=1-\dfrac{1}{50}\)

\(\Rightarrow S=\dfrac{49}{50}\)