3/2-5/6+7/12-9/20+11/30-13/42+15/56-17/72+19/90=?

原式=(2+1)/(2*1)-(3+2)/(3*2)+(4+3)/(4*3)-(5+4)/(5*4)+(6+5)/(6*5)-(6+7)/(6*7)+(7+8)/(7*8)-(8+9)/(8*9)+(9+10)/(9*10) =(1+1/2)-(1/2+1/3)+(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+1/3+1/3+1/5+1/5+1/7+1/7+1/9+1/9+1/10 =结果自己算、、、太麻烦了、