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S=1/1.3.5 +1/3.5.7+...................+1/19.21.23
---> 4S=4/1.3.5 + 4/ 3.5.7 +.........................+4/19.21.23
= (1/1.3 -1/3.5 ) + ( 1/3.5 -1/3.7) +...................................+(1/... -1/21.23 )
= 1/1.3 -1/21.23
= 1/3 -1/483 =160/483
----> S = 160/483 : 4 = 160 / 1932
(em tự rút gọn nhé! )S=1/1.3.5 +1/3.5.7+...................+1/19.21.23
---> 4S=4/1.3.5 + 4/ 3.5.7 +.........................+4/19.21.23
= (1/1.3 -1/3.5 ) + ( 1/3.5 -1/3.7) +...................................+(1/... -1/21.23 )
= 1/1.3 -1/21.23
= 1/3 -1/483 =160/483
----> S = 160/483 : 4 = 160 / 1932
(em tự rút gọn nhé! )
M=1/1.3.5 +1/3.5.7+.....+1/19.21.23
4M=4/1.3.5 + 4/ 3.5.7 +...+4/19.21.23
= (1/1.3 -1/3.5 ) + ( 1/3.5 -1/3.7) +....+(1/19.21 -1/21.23 )
= 1/1.3 -1/21.23
= 1/3 -1/483
=160/483
⇒ S = 160/483 : 4
= 160 / 1932
=40/438
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