Question

(ghi rõ từng bước cho mình nhé)D=3/2+3/6+3/12+3/20+3/42+...+3/90

Answer 1

\(D=\frac{3}{2}+\frac{3}{6}+\frac{3}{12}+...+\frac{3}{90}\)

\(D=\frac{3}{1.2}+\frac{3}{2.3}+\frac{3}{3.4}+...+\frac{3}{9.10}\)

\(D=3.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(D=3.\left(1-\frac{1}{10}\right)=3.\frac{9}{10}=\frac{27}{10}\)

Answer 2

3/1.2 + 3/2.3 + 3/3.4 + 3/4.5 + 3/6.7 hả

thiếu 3/5.6