3分之1+15分之1+35分之1+63分之1+···+17×19分之1
=1/2*[(1-1/3)+(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+……+(1/17-1/19)]
=1/2*[1-1/19]
=1/2*18/19
=9/19
解:原式=(0.5/1-0.5/3)+(0.5/3-0.5/5)+(0.5/5-0.5/7))+(0.5/7-0.5/9)+...+)+(0.5/17-0.5/19)
=0.5/1-0.5/19
=0.5*(18/19)
=9/19
我靠!!!!!太难了吧!!!!!哪个混球出的?????去扁他!!!!!