5.7+7.9+9.11+...93.95
S=2/5.7+2/7.9+2/9.11+.........+2/93.95
Bg
Ta có: S = \(\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{93.95}\)
=> S = \(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+...+\frac{1}{93}-\frac{1}{95}\)
=> S = \(\frac{1}{5}-\frac{1}{95}\)
=> S = \(\frac{19}{95}-\frac{1}{95}\)
=> S = \(\frac{18}{95}\)
Vậy S = \(\frac{18}{95}\)
\(S=\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+\frac{2}{9\cdot11}+...+\frac{2}{93\cdot95}\)
\(S=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+...+\frac{1}{93}-\frac{1}{95}\)
\(S=\left(\frac{1}{5}-\frac{1}{95}\right)+\left(\frac{-1}{7}+\frac{1}{7}\right)+\left(\frac{-1}{9}+\frac{1}{9}\right)+...+\left(\frac{-1}{93}+\frac{1}{93}\right)\)
\(S=\left(\frac{1}{5}-\frac{1}{95}\right)\)
\(S=\frac{19}{95}-\frac{1}{95}\)
\(S=\frac{18}{95}\)
Bài làm :
Ta có:
\(S=\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{93.95}\)
\(S=\frac{7-5}{5.7}+\frac{9-7}{7.9}+\frac{11-9}{9.11}+...+\frac{95-93}{93.95}\)
\(S=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+...+\frac{1}{93}-\frac{1}{95}\)
\(S=\frac{1}{5}-\frac{1}{95}\)
\(S=\frac{18}{95}\)
\(\text{Vậy S =}\frac{18}{95}\)
Chúc bạn học tốt !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Tinh nhanh:
S = 2/5.7 + 2/7.9+ 2/9.11+...+2/93.95
\(S=\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+\frac{2}{9\cdot11}+..............+\frac{2}{93\cdot95}\)
\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+............+\frac{1}{93}-\frac{1}{95}\)
\(=\frac{1}{5}-\frac{1}{95}\)
S = 2/5.7+ 2/7.9+...+2/93.95
=1/5-1/7+1/7-1/9+1/11+...+1/93+1/95
=1/5-1/95
=19/95-1/95
=18/95
tinh nhanh:s=2/5.7+2/7.9+2/9.11+...+2/93.95+3/95.98+4/98.102+5/102.107+2012/107.2119
S=\(\frac{2}{5.7}\)+\(\frac{2}{7.9}\)+....+\(\frac{2}{93.95}\)+\(\frac{3}{95.98}\)+\(\frac{4}{98.102}\)+\(\frac{5}{102.107}\)+\(\frac{2012}{107.2119}\)
S=\(\frac{1}{5}\)-\(\frac{1}{7}\)+\(\frac{1}{7}\)-\(\frac{1}{9}\)+....+\(\frac{1}{93}\)-\(\frac{1}{95}\)+\(\frac{1}{95}\)-\(\frac{1}{98}\)+\(\frac{1}{98}\)-\(\frac{1}{102}\)+\(\frac{1}{102}\)-\(\frac{1}{107}\)+\(\frac{1}{107}\)-\(\frac{1}{2119}\)
S=\(\frac{1}{5}\)-\(\frac{1}{2119}\)
S=\(\frac{2114}{10595}\)
S= 2/5.7 + 2/7.9 + 2/9.11 + .... + 2/93.95 + 3/95.98 + 4/98.102 + 5/102.107 + 2012/107.2119
Mình cần gấp lắm ! Trước 8h mình phải nộp bài cho cô giáo rồi. Giúp mình nha thanks
Khoảng cách có rồi thì bạn áp dụng công thức : \(\frac{a}{m.n}=\frac{1}{m}-\frac{1}{n}\)(với n-m=a) là làm được
S=\(\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{93.95}+\frac{3}{95.98}+\frac{4}{98.102}+\frac{5}{102.17}+\frac{2012}{107..2119}\)
S=\(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{93}-\frac{1}{95}+\frac{1}{95}-\frac{1}{98}+\frac{1}{98}-\frac{1}{102}+\frac{1}{102}-\frac{1}{107}+\frac{1}{107}-\frac{1}{2119}\)
S=\(\frac{1}{5}-\frac{1}{2119}\)
S=\(\frac{2114}{10595}\)
5.7+7.9+9.11+...+25.27
5.7+7.9+9.11+....+25.27
5.7+7.9+9.11+...+25.27
=5.(5+2)+7.(7+2)+9.(9+2)+...+25.(25+2)
=52+2.5+72+2.7+92+2.9+...+252+2.25
=(52+72+92+...+252)+(2.5+2.7+2.9+...+2.25)
=2915+2.(5+7+9+...+25)
=2915+2.165
=2915+330
=3245
Tính:
a) M=2/3.5+2/5.7+2/7.9+...+2/97.99
b) N=3/5.7+3/7.9+3/9.11+...+3/197.199
a.
\(M=1.\left[\frac{1}{3}-\frac{1}{5}+.....\frac{1}{97}-\frac{1}{99}\right]\)
\(M=\frac{1}{3}-\frac{1}{99}=\frac{32}{99}\)
b.
\(N=\frac{3}{2}.\left[\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{197}-\frac{1}{199}\right]\)
\(N=\frac{3}{2}.\left[\frac{1}{5}-\frac{1}{199}\right]=\frac{291}{995}\)
mk đầu tiên nha bạn
4/5.7+4/7.9+4/9.11+...+4/53.55 = ?
\(=4\left(\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+...+\frac{1}{53.55}\right)\)
\(=4\left(\frac{1}{5}-\frac{1}{5}+\frac{1}{7}-\frac{1}{7}+...+\frac{1}{53}-\frac{1}{55}\right)\)
\(=4\left(\frac{1}{5}-\frac{1}{55}\right)\)
\(=4.\frac{2}{11}\)
\(=\frac{8}{11}\)
11/5.7+11/7.9+11/9.11+......+11/59.61