B=5/1.5+5/5.9+5/9.13+...........+5/101.105
=(4/1.5+4/5.9+4/9.13+..........+4/101.105).5/4
=(1/1-1/5+1/5-1/9+1/9-1/13+..........+1/101-1/105).5/4
=(1/1-1/105).5/4
=104/105.5/4
=26/21
\(B=\frac{5}{1.5}+\frac{5}{5.9}+\frac{5}{9.13}+.......+\frac{5}{101.105}\)
\(B=5.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+.........+\frac{1}{101}-\frac{1}{105}\right)\)
\(B=5.\left(1-\frac{1}{105}\right)\)
\(B=\frac{5.104}{105}=\frac{104}{21}\)
nha m.n!
\(b=\frac{5}{4}\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}...+\frac{1}{101}-\frac{1}{105}\right)\)
\(=\frac{5}{4}\left(1-\frac{1}{101}\right)\)
\(=\frac{5}{4}\cdot\frac{100}{101}\)
\(=\frac{125}{101}\)
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