Question

tính tổng

A= -1/20 + -1/30 + -1/42 + -1/56 + -1/72 + -1/90

Answer 1

TA có:

\(A=\left(-1\right).\left(\frac{1}{20}+\frac{1}{30}+...+\frac{1}{90}\right)\)

\(=\left(-1\right).\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\right)\)

\(=\left(-1\right).\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}+\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(=\left(-1\right).\left(\frac{1}{4}-\frac{1}{10}\right)=\left(-1\right).\left(\frac{5}{20}-\frac{2}{20}\right)=\left(-1\right).\left(\frac{3}{20}\right)=-\frac{3}{20}\)

Answer 2

-A= \(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)=\(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)

=\(\frac{1}{4}-\frac{1}{10}\)=\(\frac{3}{20}\)