Chứng minh rằng: \(\frac{1}{5}+\frac{1}{13}+\frac{1}{25}+.....+\frac{1}{1985}< \frac{9}{20}\)
so sánh A= \(\frac{3^{2003}+5}{3^{2001}+5}\)và B= \(\frac{3^{2001}+1}{3^{1009}+1}\)
so sánh A= \(\frac{1}{1.1981}+\frac{1}{2.1982}+......+\frac{1}{24.2004}+\frac{1}{25.2005}\)
B =\(\frac{1}{1.61}+\frac{1}{2.27}+.....+\frac{1}{1979.2004}+\frac{1}{1980.2005}\)
Các bn lm giúp mk nhé (dấu . là dấu x nhé )