Bài 7
a)1/1.2+1/2.3+1/3.4+...+1/99.100
=1/1-1/2+1/2-1/3+1/3-1/4+....+1/99-1/100
=100/100-1/100=99/100
b)(1-1/2)(1-1/3).....(1-1/n+1)
=1/2.2/3.......n/n+1
=1/n+1
c)-66(1/2-1/3+1/11) + 124(-37)+63(-124)
=(-66).17/66 - 124 (37+63)
=-17-124.100=-17-12400=-12417
d)7/4(33/12+3333/2020+333333/303030+33333333/42424242)
=7/4(33/12+33.101/20.101+33.10101/30.10101+33.1010101/42.1010101)
=7/4(33/12+33/20+33/30+33/42)
=33.7/4(1/12+1/20+1/30+1/42)
=231/4.(1/3.4+1/4.5+1/5.6+1/6.7)
=231/4(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7)
=231/4(1/3-1/7) = 231/4 . 4/21 = 231/11 = 21
