Question

A Cepheid variable star is a star whose brightness alternately increases and decreases. For a certain star, the interval between times of maximum brightness is 5.7 days. The average brightness of this star is 5.0 and its brightness changes by ±0.25. In view of these data, the brightness of the star at time t, where t is measured in days, has been modeled by the function B(t) = 5.0 + 0.25 sin 2πt 5.7 .Find the rate of change of the brightness after t days.

Answer 1

Correct expression of B(t) is;

B(t) = 5.0 + 0.25 sin(2πt/5.7)

Answer:

B'(t) = (5π/57)cos(2πt/5.7)

Explanation:

We are given;

B(t) = 5.0 + 0.25 sin(2πt/5.7)

Now the rate of change of the brightness after t days is simply the derivative of B(t)

Thus;

B'(t) = 0 + [{0.25 cos(2πt/5.7)} × (2π/5.7)]

This leads to;

B'(t) = (0.5π/5.7)cos (2πt/5.7)

Simplifying this further gives;

B'(t) = (5π/57)cos(2πt/5.7)