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Đặt A=\(\frac{1}{3}.5+\frac{1}{5}.7+...+\frac{1}{97}.99\)
=>A=\(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{97.99}\)
=>2A=\(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{97.99}\)
=>2A=\(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{97}-\frac{1}{99}\)
=>2A=\(\frac{1}{3}-\frac{1}{99}=\frac{33}{99}-\frac{1}{99}=\frac{32}{99}\)
=>A=\(\frac{32}{99}:2=\frac{32}{99}.\frac{1}{2}=\frac{32}{198}=\frac{16}{99}\)
a; \(\dfrac{3}{5}\) - \(\dfrac{-7}{10}\) - \(\dfrac{13}{-20}\)
= \(\dfrac{12}{20}\) + \(\dfrac{14}{20}\) + \(\dfrac{13}{20}\)
= \(\dfrac{39}{20}\)
b; \(\dfrac{1}{2}\) + \(\dfrac{1}{-3}\) + \(\dfrac{1}{4}\) - \(\dfrac{-1}{6}\)
= \(\dfrac{6}{12}\) - \(\dfrac{4}{12}\) + \(\dfrac{3}{12}\) + \(\dfrac{2}{12}\)
= \(\dfrac{7}{12}\)
\(75\%-1\frac{1}{2}+0,5:\frac{5}{12}-\frac{-1^2}{2}\)
\(=\frac{75}{100}-\frac{3}{2}+\frac{5}{10}.\frac{12}{5}-\frac{1}{2}\)
\(=\frac{3}{4}-\frac{3}{2}+\frac{6}{5}-\frac{1}{2}\)
\(=\frac{15}{20}-\frac{30}{20}+\frac{24}{20}-\frac{10}{20}\)
\(=\frac{15-30+24-10}{20}\)
\(=\frac{-1}{20}\).
4/5 . 5/8 + 1/8 . 4/5 - 4/5 . -1/4
= 4/5.( 5/8 + 1/8 -(-1/4)
= 4/5. 1
= 4/5
Hok tốt &_&
\(\frac{4}{5}\)X \(\frac{5}{8}\)+\(\frac{1}{8}\)X\(\frac{4}{5}\)-\(\frac{4}{5}\)X\(\frac{-1}{4}\)
=4/5x(5/8+1/8-(-1/4)=4/5x5/4=1
a, 6/5
b,1