`5.`
` a, 7/23 + 4/17 - 7/23 +13/17`
` = (7/23 - 7/23 ) +(4/17 +13/17)`
` = 0 + 17/17 = 0+1=1`
` b,19(1)/(4).(7)/(12) -15(1)/(4).(7)/(12)`
` = (7)/(12).(19(1)/(4) -15(1)/(4) )`
` = (7)/(12).4 = (7.4)/(3.4)`
` =(7)/3`
`c, (1 -(1)/(2) ) .(1-(1)/(3) ).....(1-(1)/(2012) )`
Ta có `:`
` 1- 1/2 =1/2`
` 1 -1/3 = 2/3`
` ....`
` 1 - 1/2012 = 2011/2012`
` => 1/2 . 2/3 . 3/4 ..... 2011/2012`
` = (1.2.3.....2011)/(2.3.4....2012)`
` = 1/2012`
