Răspuns:
4
Explicație pas cu pas:
N=√a-b
a=6/5+7/10+8/15+...+85/400=1/5(6+7/2+8/3+....+85/80)
b=1/5(5+5/2+5/3+......+5/80)
N=√1/5(6+7/2+8/3+....+85/80)-1/5(5+5/2+5/3+......+5/80)
=√1/5[(6-5)+(7/2-5/2)+(8/3-5/3)+......+(85/80-5/80)]=
=√1/5(1+1+1+....+1)
......80 ori...
=√1/5×80=√16=4