>>502
行列式det使うと簡単
> po <- c(1/2, sqrt(3)/6, sqrt(2/3))
> pa <- c(0,0,0)
> pb <- c(1,0,0)
> pc <- c(cos(pi/3), sin(pi/3), 0)
> det(cbind(pa-po,pb-po,pc-po))/6
[1] -0.1178511