数学题7/12-9/20+11/30-13/42+15/56-17/72+19/90怎样做?

=(3+4)/3*4 - (4+5)/4*5 + (5+6)/5*6 - (6+7)/6*7 +(7+8)/7*8 -(8+9)/8*9 +(9+10)/9*10
=3/3*4+4/3*4 - 4/4*5 -5/4*5 .+9/9*10+10/9*10
=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8-1/8-1/9+1/9+1/10
=1/3+1/10
=13/30