Toán tính nhanh:5^2/1.3+5^2/3.5+5^2/5.7+5^2/7.9+…..+5^2/103+105 11/09/2021 By Allison tính nhanh:5^2/1.3+5^2/3.5+5^2/5.7+5^2/7.9+…..+5^2/103+105
`5^2/1.3 + 5^2/3.5 + 5^2/5.7 + …. + 5^2/103.105` `= ( 2/1.3 + 2/3.5 + 2/5.7 + …. + 2/103.105 ) : 2 . 5^2` `= ( 1/1 – 1/3 + 1/3 – 1/5 + 1/5 – 1/7 + … + 1/103 – 1/105 ) : 2 . 25` `= ( 1/1 – 1/105 ) : 2 . 25` `= 104/105 : 2 . 25` `= 52/105 . 25` `= 260/21` Trả lời
Giải thik nè :nhóm 5 ra ngoài TC 5.(2/1.3+2/3.5+2/5.7+2/7.9+…..+2/103.105) =5.(1-1/3+1/3-1/5+…+1/103-1/105) =104/21 Trả lời
`5^2/1.3 + 5^2/3.5 + 5^2/5.7 + …. + 5^2/103.105`
`= ( 2/1.3 + 2/3.5 + 2/5.7 + …. + 2/103.105 ) : 2 . 5^2`
`= ( 1/1 – 1/3 + 1/3 – 1/5 + 1/5 – 1/7 + … + 1/103 – 1/105 ) : 2 . 25`
`= ( 1/1 – 1/105 ) : 2 . 25`
`= 104/105 : 2 . 25`
`= 52/105 . 25`
`= 260/21`
Giải thik nè :nhóm 5 ra ngoài
TC 5.(2/1.3+2/3.5+2/5.7+2/7.9+…..+2/103.105)
=5.(1-1/3+1/3-1/5+…+1/103-1/105)
=104/21