Vă rog frumos!!!!!!!!!!!!!

Răspuns:
Explicație pas cu pas:
a=1/2·3/4·5/6.......19/20
b=2/3·4/5·6/7.........20/21
a·b=1/2·3/4·5/6.......19/20·2/3·4/5·6/7.......20/21=1/21