xr1 = c(20.5, 27, 15.5, 18.2)
xr2 = c(17.5, 13.5, 13.8, 19.9)
mean(xr2)
sd(xr2)


tr <- function(xr)  sqrt( 2 * (xr/100) / 9.80)


tr(xr1)
tr(xr2)
mean(tr(xr2))
sd(tr(xr2))

t1 = tr(xr1)
t2 = tr(xr2)
mean(t2)
sd(t2)


mia.media <- function(x) sum(x)/length(X)
mia.media(t2)


mia.sd <- function(x) sqrt( sum( (x-mia.media(x))^2 ) / length(x) )
mia.sd(t2)

mia.sd1 <- function(x) sqrt( sum( (x-mia.media(x))^2 ) / (length(x)-1) )
mia.sd1(t2)