The illuminance \(I\), measured in lux, from an LED fixture is modeled by \(I(d)=\frac{800}{(d+2)^2}\), where \(d\ge0\) is the distance from the fixture in meters.
a) Find \(I(0)\) and interpret the value in context.
b) Expand the denominator, then use the quotient rule. Write the quotient-rule expression for \(I'(d)\) before simplifying it. Simplify \(I'(d)\) and determine its sign for \(d\ge0\).
c) Find \(\lim_{d\to\infty}I(d)\) and interpret the result.
Hints
- Expand the squared denominator so differentiating it does not require a later rule.
- In the quotient rule, identify the numerator and denominator functions before substituting their derivatives.
- Determine the sign only after simplifying the derivative.
- Interpret the limiting illuminance using the model's units.
Solution
1. \(I(0)=\frac{800}{4}=200\), so the modeled illuminance at \(d=0\) is \(200\,\text{lx}\).
2. Expand \((d+2)^2=d^2+4d+4\). With \(u(d)=800\) and \(v(d)=d^2+4d+4\), \(u'(d)=0\) and \(v'(d)=2d+4\).
3. The quotient-rule expression is \(I'(d)=\frac{0\cdot(d^2+4d+4)-800(2d+4)}{(d^2+4d+4)^2}\).
4. Simplifying gives \(I'(d)=-\frac{1600}{(d+2)^3}\). For \(d\ge0\), the denominator is positive, so \(I'(d)<0\).
5. As \(d\to\infty\), the denominator grows without bound, so \(I(d)\to0\). In the model, illuminance approaches \(0\,\text{lx}\) at very large distances.
Answer
a) \(200\,\text{lx}\); this is the modeled illuminance at \(d=0\)
b) \(I'(d)=\frac{0\cdot(d^2+4d+4)-800(2d+4)}{(d^2+4d+4)^2}=-\frac{1600}{(d+2)^3}<0\) for \(d\ge0\)
c) \(\lim_{d\to\infty}I(d)=0\); the modeled illuminance approaches \(0\,\text{lx}\)