老师上课 没认真听 1
1.1/2+1/6+1/12+1/20+...+1/90
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-/10
=1-1/10=9/10
2.1/2-1/6-1/12-1/20-1/30-1/42-...-1/132
=1-1/2-1/6-1/12-1/20-1/30-1/42-...-1/132
=1-(1/2+1/6+1/12+1/20+...+1/132)
=1-(1-1/2+1/2-1/3+1/3-1/4+1/5-1/6=...+1/11-1/12)
=1-(1-1/12)=1/12
2
错位相减
1.……=1-1/10=1/10
2.……=1-1/12=11/12
怎么样 我牛吧?