Tìm tỷ số của A và B biết rằng:
A= 1/1*1981+1/2*1982+...+1/n*(1980+n)+...+1/25*2005
B=1/1*26=1/1*27+...+1/m*(25+m)+...+1/1980*2005
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NT
Nguyễn Thị Thương Hoài
Giáo viên
VIP
12 tháng 8 2023
D = \(\dfrac{1}{1\times1981}\) + \(\dfrac{1}{2\times1982}\)+...+ \(\dfrac{1}{25\times2005}\)
D =\(\dfrac{1}{1980}\times\)( \(\dfrac{1980}{1\times1981}\)+ \(\dfrac{1980}{2\times1982}\)+....+ \(\dfrac{1980}{25\times2005}\))
D = \(\dfrac{1}{1980}\) \(\times\)(\(\dfrac{1}{1}\) - \(\dfrac{1}{1981}\) + \(\dfrac{1}{2}\) - \(\dfrac{1}{1982}\)+....+ \(\dfrac{1}{25}\) \(\times\) \(\dfrac{1}{2005}\))
D= \(\dfrac{1}{1980}\)[( \(\dfrac{1}{1}\) + \(\dfrac{1}{2}\) +....+ \(\dfrac{1}{25}\)) - ( \(\dfrac{1}{1981}\)+ \(\dfrac{1}{1982}\)+...+ \(\dfrac{1}{2005}\))]
E =\(\dfrac{1}{25}\times\)( \(\dfrac{1}{1\times26}\)+ \(\dfrac{1}{2\times27}\)+...+ \(\dfrac{1}{1980\times2005}\))
E = \(\dfrac{1}{25}\). (\(\dfrac{25}{1\times26}\) + \(\dfrac{25}{2\times27}\)+....+ \(\dfrac{25}{1980\times2005}\))
E = \(\dfrac{1}{25}\).(\(\dfrac{1}{1}\)-\(\dfrac{1}{26}\)+\(\dfrac{1}{2}\)-\(\dfrac{1}{27}\)+...+\(\dfrac{1}{1980}\)-\(\dfrac{1}{2005}\))
E=\(\dfrac{1}{25}\)[\(\dfrac{1}{1}\)+...+ \(\dfrac{1}{25}\)+ (\(\dfrac{1}{26}\)+...+\(\dfrac{1}{1980}\)) - (\(\dfrac{1}{26}\)+...+\(\dfrac{1}{1980}\)) - (\(\dfrac{1}{1981}\)+..\(\dfrac{1}{2005}\))]
E = \(\dfrac{1}{25}\) .[\(\dfrac{1}{1}\)+\(\dfrac{1}{2}\)+...+\(\dfrac{1}{25}\) - (\(\dfrac{1}{1981}\)+\(\dfrac{1}{1982}\)+...+ \(\dfrac{1}{2005}\))]
\(\dfrac{D}{E}\) = \(\dfrac{\dfrac{1}{1980}}{\dfrac{1}{25}}\) = \(\dfrac{5}{396}\)
2 tháng 3 2018
25B=\(\frac{25}{1.26}+\frac{25}{2.27}+.......+\frac{25}{1980.2005}\)
\(=1-\frac{1}{26}+\frac{1}{2}-\frac{1}{27}+..........+\frac{1}{1980}-\frac{1}{2005}\)
\(=1+\frac{1}{2}+\frac{1}{3}+.....+\frac{1}{25}-\frac{1}{1981}-\frac{1}{1982}-....-\frac{1}{2005}\)
1980A=\(\frac{1980}{1.1981}+\frac{1980}{2.1982}+.........+\frac{1980}{25.2005}\)
\(=1-\frac{1}{1981}+\frac{1}{2}-\frac{1}{1982}+......+\frac{1}{25}-\frac{1}{2005}\)
\(=1+\frac{1}{2}+...+\frac{1}{25}-\frac{1}{1981}-\frac{1}{1982}-...-\frac{1}{2005}\)
\(\Rightarrow25B=1980A\)\(\Rightarrow\frac{A}{B}=\frac{25}{1980}\)
