A, 1/4.5 + 1/5.6 + 1/6.7+ 1/7.8
= 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8
= 1/4 - 1/8
=1/8
B; 3/1.4 + 3/4.7 + 3/7.10 = 1/1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10
= 1- 1/10
= 9/10
C, 1/1.3 + 1/3.5 + 1/5.7 + .. +1/2007.2009
= 1/2 ( 2/1.3 + 2/3.5 + ... + 2/2007.2009)
= 1/2 ( 1/1 - 1/3 + 1/3 - 1/5 + ... +1/2007 - 1/2009)
= 1/2 ( 1- 1/2009)
= 1/2 . 2008/2009
= 1004/2009
D; 8/2.6 + 8/6.10 + 8/10.14 + 8/14.18 + 8/18.22
= 2( 4/2.6 + 4/6.10 + .. +4/18.22)
= 2 ( 1/2 - 1/6 + 1/6 - 1/10 + .. + 1/18 - 1/22)
= 2 ( 1/2 - 1/22)
= 2 .5/11
= 10/11
E; 3/2.4 + 3/4.6 + ... +3/998.1000
= 3/2( 2/2.4 + 2/4.6 +.. +2/998.1000)
= 3/2 .( 1/2 - 1/4 + 1/4 - 1/6 + ... +1/998 - 1/1000 )
=3/2 . 449/1000
= 1497/2000
ẤN đúng cho mình nha làm xong đứt hơi
a ) 1/4.5 + 1/5.6 + 1/6.7+ 1/7.8
= 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8
= 1/4 - 1/8
=1/8
b ) 3/1.4 + 3/4.7 + 3/7.10 = 1/1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10
= 1- 1/10
= 9/10
c ) 1/1.3 + 1/3.5 + 1/5.7 + .. +1/2007.2009
= 1/2 ( 2/1.3 + 2/3.5 + ... + 2/2007.2009)
= 1/2 ( 1/1 - 1/3 + 1/3 - 1/5 + ... +1/2007 - 1/2009)
= 1/2 ( 1- 1/2009)
= 1/2 . 2008/2009
= 1004/2009
d ) 8/2.6 + 8/6.10 + 8/10.14 + 8/14.18 + 8/18.22
= 2( 4/2.6 + 4/6.10 + .. +4/18.22)
= 2 ( 1/2 - 1/6 + 1/6 - 1/10 + .. + 1/18 - 1/22)
= 2 ( 1/2 - 1/22)
= 2 .5/11
= 10/11
e ) 3/2.4 + 3/4.6 + ... +3/998.1000
= 3/2( 2/2.4 + 2/4.6 +.. +2/998.1000)
= 3/2 .( 1/2 - 1/4 + 1/4 - 1/6 + ... +1/998 - 1/1000 )
=3/2 . 449/1000
= 1497/2000
