x <- seq(0,7,length=100)
fx <- df(x,df1=4, df2 = 2)
lb = 2
ub = 4

plot(x, fx, type="l", xlab="", ylab="")

i <- x >= lb & x <= ub
lines(x, fx)
polygon(c(lb,x[i],ub), c(0,fx[i],0), col="red")

area <- pf(ub,df1=4, df2=2) - pf(lb,df1=4, df2 = 2)
result <- paste("P(",lb,"< X <",ub,") =", signif(area, digits=3))

mtext(result,3)