Question

Tính :

\(B=\frac{3}{2\cdot5}+\frac{3}{5\cdot8}+\frac{3}{8\cdot11}+...+\frac{3}{23\cdot26}\)

9 tk danh cho nguoi đầu tiên

Answer 1

\(B=\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+...+\frac{3}{23.26}\)

\(B=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+....+\frac{1}{23}-\frac{1}{26}\)

\(B=\frac{1}{2}-\frac{1}{26}\)

\(B=\frac{13}{26}-\frac{1}{26}\)

\(B=\frac{12}{26}=\frac{4}{13}\)

Answer 2

Ta có: 3/2.5=3/3.(1/2-1/5)

3/5.8=3/3.(1/5-1/8)

3/8.11=3/3.(1/8-1/11)

...............................

3/23.26=3/3.(1/23-1/26)

Cộng từng vế ta đc:

B=3/3.(1/5-1/26)=1.21/130=21/130

Answer 3

B = 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - ... - 1/26

B = 1/2 - 1/26

B = 6/13 nha bạn

Answer 4

mình tính nhầm nhé\(\frac{12}{26}=\frac{6}{13}\)