0

0

원식=1+1/(2*3)+1+1/(3*4)+1+1/(4*5)+.+1+1/(20*21)
=19 + (1/2 -1/3) + (1/3 -1/4) +(1/4 -1/5) + .+ (1/20 -1/21)
= 19 + 1/2 -1/21
19 와 19/42