diff --git a/ch04.r b/ch04.r
index fd93c98..98c4c9d 100644
--- a/ch04.r
+++ b/ch04.r
@@ -57,3 +57,29 @@ V = unifstat(a, b)$var
 a = 0; b = 1;
 F = punif(0.3, a, b) - punif(0.2, a, b)
 
+
+## PDF and CDF of an exponential random variable
+
+# R code to plot the exponential PDF
+lambda1 = 1/2
+lambda2 = 1/5
+x = seq(0, 1, (0+1)/1000)
+f1 = dexp(x, 1/lambda1)
+f2 = dexp(x, 1/lambda2)
+plot(x, f2, type="n")
+lines(x, f1, lwd=4, col="blue")
+lines(x, f2, lwd=4, col="red")
+legend(0, 1, legend=c(expression(paste(lambda, "=5")), expression(paste(lambda, "=2"))), col=c("red", "blue"), lty=1:1)
+
+# R code to plot the exponential CDF
+lambda1 = 1/2
+lambda2 = 1/5
+x = seq(0, 1, (0+1)/1000)
+F1 = pexp(x, 1/lambda1)
+F2 = pexp(x, 1/lambda2)
+plot(x, F2, type="n")
+lines(x, F1, lwd=4, col="blue")
+lines(x, F2, lwd=4, col="red")
+legend(0, 1, legend=c(expression(paste(lambda, "=5")), expression(paste(lambda, "=2"))), col=c("red", "blue"), lty=1:1)
+
+