1-1/2=(2-1)/1*2=1/1*2
1/2-1/3=(3-2)/2*3=1/2*3
1/3-1/4=(4-3)/3*4=1/3*4
1/4-1/5=(5-4)/4*5=1/4*5
.
1/39-1/40=(40-39)/39*40=1/39*40
所以
1/1*2+1/2*3+1/3*4+1/4*5……+1/39*40
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+.+(1/39-1/40)
=1-1/40
=39/40
1-1/2=(2-1)/1*2=1/1*2
1/2-1/3=(3-2)/2*3=1/2*3
1/3-1/4=(4-3)/3*4=1/3*4
1/4-1/5=(5-4)/4*5=1/4*5
.
1/39-1/40=(40-39)/39*40=1/39*40
所以
1/1*2+1/2*3+1/3*4+1/4*5……+1/39*40
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+.+(1/39-1/40)
=1-1/40
=39/40