Question

Tính C = 1/1.2.3 + 1/2.3.4 +...+1/38.39.40

Answer 1

Bạn ấn vào dòng chữ màu xanh Cho D =  1/1.2.3+1/2.3.4+....+1/37.38.39

Answer 2

\(C=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{38.39.40}=\frac{1}{2}\cdot\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{38.39}-\frac{1}{39.40}\right)=\frac{1}{2}\cdot\left(\frac{1}{2}-\frac{1}{39.40}\right)=\frac{1}{2}\cdot\frac{599}{1200}=\frac{599}{2400}\)