Question

A photographer uses a light meter to measure the intensity of light from a flashbulb. The intensity for the flashbulb, I, in lux, is a function of the distance from the light, d, in metres, and can be represented by I(d)=10/d^2,d>0
a) Determine the following, to two decimal places.
i) the intensity of light 3 m from the flash bulb
ii) the average rate of change in the intensity of light for the indicated 1<d<3
iii) the approximate instantaneous rate of change in the intensity of light at exactly 3 m from the flash bulb
b) What does the sign of your answer to part a), subpart iii), indicate about the light intensity?

Answer 1

Here is what I have done
ai) l(d)=10/d^2
l(3)=10/3^2
l(3)=1.11

aii) AROC = y2-y1/x2-x1
=10-1.1111/1-3
=8.8889/-2
=-4.44445
=-4.44
I just need help on aiii) and b