20/sqrt(20)+16/sqrt(80)-sqrt(1/5)=20/sqrt(20)+16/sqrt(80)-1/sqrt(5)=20sqrt(20)/20+16sqrt(80)/80-sqrt(5)/5=(80sqrt(20)+16sqrt(80)-16sqrt(5))/80=(160sqrt(5)+64sqrt(5)-16sqrt(5))/80=208sqrt(5)/80=13sqrt(5)/5=13/sqrt(5).
Ответ:D)13/sqrt(5).
Sqrt-обозначение квадратного корня.