Albert, astronomy amateur, notices that the brightness of Delta Cephei periodically increases and decreases. He wonders how fast does its brightness changes at time equals 1day?

What is known:

• The brightness cycles every 5 days
• The brightness of the star is modeled by the function:
  B(t) = 4 + 0.35 sin(2πt/b)
• The time, t, is 1 day.

How fast is the brightness changing after a day?

Applying the chain rule to find the derivative of the brightness.