Răspuns:
4L/99= 2017/99
Explicație pas cu pas:
2021(1/4*5+1/5*6+1/6*7+....+1/2020*2021)
tinem cont ca 1/k(k+1)=1/k-1/(k+1)
paranteza devine
1/4-1/5+1/5-1/6+...+1/2020-1/2021=1/4-1/2021=(2021-4)/4*2021=2017/4*2021
si L devine 2017/4
4L=2017
4L/99= 2017/99