//Example 7
//chapter 42
//clc()
n=1.65
r_1=40//in cm
r_2=-40//in cm
disp("x=1/f in cm=")
x=(n-1)*((1/r_1)-(1/r_2))
disp(x)
disp("f=1/x")
f=1/x
disp(f,"f in cm=")