1/ Tính nhanh:
a) 1/1.1/2+1/2.1/3+1/3.1/4+...+1/8.1/9+1/9.1/10
b) 1/6+1/12+1/20+1/30+...+1/90+1/110
c) 1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/100+1/132
d) 3/3^2 - 8/3^2 . 15/4^2 . .... 899/30^2
2/Chứng minh rằng:
a) 1/101+1/102+...+1/199+1/200 < 1
b) 1/2^2+1/3^2+...+1/2012^2+1/2013^2 < 1