A=5/20+5/21+5/22+...+5/49 chứng minh rằng 3<A<8
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\(A=\left(\frac{7}{9}+1\right)\left(\frac{7}{20}+1\right)\left(\frac{7}{33}+1\right)..........\left(\frac{7}{2900}+1\right)\)
\(A=\frac{16}{9}.\frac{27}{20}.\frac{40}{33}.......\frac{2907}{2900}\)
\(A=\frac{2.8}{1.9}.\frac{3.9}{2.10}.\frac{4.10}{3.11}......\frac{51.57}{50.58}\)
\(A=\frac{2.3.4.....51}{1.2.3......50}.\frac{8.9.10.......57}{9.10.11........58}\)
\(A=51.\frac{4}{29}\)
\(A=\frac{204}{59}\)
Vậy \(A=\frac{204}{59}\)
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32015=32014.3=(32)1007.3=A91007.3=B9.3=(...7)
Tận cùng 7.
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a) 1/1001+ 1/1002+ ... + 1/2500> 1/5
Ta có: 1/1001> 1/2500+ 1/1002> 1/2500+ ...+1/2500 =1/2500
=>( 1/1001+ 1/1002+ ...+ 1/2500)> ( 1/2500+ 1/2500+ ...+1/2500)
=>( 1/1001+ 1/1002+ ...+ 1/2500)> 3/5> 1/5
=> 1/1001+ 1/1002+ ...+ 1/2500> 1/5
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