Inverse variation

Question

The intensity I of light received from a source varies inversely as the square of the distance d from the source. If the light intensity is 5 foot-candles at 15 feet, find the light intensity at 16 feet.

Answer 1

I/d squared=5/15squared=x/16 squared----------so 5/225=x/256 and x=5.6888888888888888888888

Answer 2

i₁ d₁² = i₂d₂²

 

5(15)² = i(16)²

5(225) =256i

1125 = 256i

4.39453125 ≈ i