Recall that the surface brightness of an exponential disk has the form I(R)= I0e^(-R/hR)where I0 is the central surface brightness and hR is the disk scale length. The total brightness is given by integrating this pro le from R = 0 to R = infinity:I(total)= Integral (o to infinity) I(R) 2 pie RdR . Brightness of the exponential disk is
I(total)= 2 pie I0hR^2
Show what fraction of the total light is within one disk scale length (R < hR) and within three disk scale lengths (R < 3hR)?