1/3 = 1 / ( 2-1 )( 2+1 ) = 1/2 ( 1 - 1/3 )1/8 = 1 / ( 3-1 )( 3+1 ) = 1/2 ( 1/2 - 1/4)1/15 = 1 / ( 4-1 )( 4+1 ) = 1/2 ( 1/3 - 1/5 )1/n = 1/ ( n+1 - 1)( n+1+1)=1/2( 1/n-1/n+2 ) 后面自己算了
