Đáp án:
c,`1/(1.3)+1/(3.5)+1/(5.7)+1/(7.9)+1/(9.11)`
`=1/2. (2/(1.3)+2/(3.5)+2/(5.7)+2/(7.9)+2/(9.11))`
`=1/2.(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)`
`=1/2.(1-1/11)`
`=1/2 . 10/11=5/11`
d,`2/(1.3)+2/(3.5)+2/(5.7)+2/(7.9)+2/(9.11)`
`=1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11`
`=1-1/11`
`=10/11`
e,`(1+1/2).(1+ 1/3). ( 1+1/4)...(1+ 1/100)`
`=3/2 . 4/3 . 5/4 ................ 101/100`
`=(3.4.5....101)/(2.3.4.....100)`
`=101/2`
f/ `( 1-1/2). ( 1-1/3).( 1-1/4) ...(1-1/100)`
`=1/2 . 2/3 . 3/4 ........... 99/100`
`=(1.2.3...99)/(2.3.4....100)`
`=1/100`
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