Question

Tính tổng:

 a) A = \(\frac{1}{2}\)+ \(\frac{1}{2.3}\)+ \(\frac{1}{3.4}\)+ .... + \(\frac{1}{23.24}\)

 b) B = \(\frac{6}{15.18}\)+ \(\frac{6}{18.21}\)+ \(\frac{6}{21.24}\)+ .... + \(\frac{6}{87.90}\)

Answer 1

a,A=\(\frac{1}{2}+\frac{1}{2.3}+\frac{1}{3.4}+.........+\frac{1}{23.24}\)

A=\(\frac{1}{2}+\frac{2}{1}-\frac{1}{3}+\frac{3}{1}-\frac{1}{4}+......\frac{23}{1}-\frac{1}{24}\)

A=\(\frac{1}{2}-\frac{1}{24}\)

A=\(\frac{11}{24}\)

Answer 2

Còn câu b bạn??

Answer 3

b)\(B=2.\left(\frac{3}{15.18}+\frac{3}{18.21}+...+\frac{3}{87.90}\right)\)

\(=3.\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{87}-\frac{1}{90}\right)\)

\(=3.\left(\frac{1}{15}-\frac{1}{90}\right)\)

\(=3.\frac{5}{90}\)

\(=\frac{5}{30}\)

\(=\frac{1}{6}\)