2×4/1+4×6/1+6×8/1+……+98×100/1
等于多少?
1/(2*4)+1/(4*6)+......+1/(98*100)=1/2 [ (4-2)/(2*4)+(6-4)/(4*6)+......(100-98)/(98*100) ]
=1/2 [ 1/2- 1/4 +1/4 - 1/6 + .... +1/98-1/100 ] =1/2 ( 1/2 -1/100 ) =49/200