Use
the upside down drag curve graph
At a given mass best glide angle is at Vmd and the
corresponding IAS for that weight.
If we now lower the mass but keep the speed for the original mass we will not be
at the best angle of attack for best range in the glide, the descent angle is
steeper.
ROD = SINTHETA x TAS
Theta the glide angle is steeper and the effective IAS/TAS is higher than the
new IAS/TAS at the lower weight, therefore ROD increases.