SOLUTION: the intensity of light, l, received at a source varies inversely as the square of the the distance, d, from the source. if the light intensity is 79 foot-candles at 30 feet, find t

Algebra ->  Customizable Word Problem Solvers  -> Misc -> SOLUTION: the intensity of light, l, received at a source varies inversely as the square of the the distance, d, from the source. if the light intensity is 79 foot-candles at 30 feet, find t      Log On

Ad: Over 600 Algebra Word Problems at edhelper.com


    

Question 666455: the intensity of light, l, received at a source varies inversely as the square of the the distance, d, from the source. if the light intensity is 79 foot-candles at 30 feet, find the light intensity at 10 feet
Answer by DrBeeee(684) About Me  (Show Source):
You can put this solution on YOUR website!
Use
(1) I = c/(d^2), where I is the intensity in foot-candles, c a constant of propotionality and d is the distance in feet.
First we find c using the known intensity at a known distance or
(2) I = c/30^2 = 79, giving
(3) c = 79*900
To find the intensity at 10 feet use (1) with c of (3) and get
(4) I at 10' = 79*900/10^2 or
(5) I at 10' = 79*9 = 711
Answer: The light intensity at 10' is 711 foot-candles.