Euclidean distance for more than 2 datapoints
The Euclidean distance between 2 vectors 

$\mathbf{u}$: $(u_1, u_2, ... , u_n)$ and $\mathbf{v}$: $(v_1, v_2, ... , v_n)$ 

is simply 

$\sqrt{(u_1-v_1)^2+(u_2-v_2)^2+...+(u_n-v_n)^2}$