(a) For what values of k does the function y = cos(kt) satisfy the differential equation 81y'' = −4y? (Enter your answers as a comma-separated list.) k = Correct: Your answer is correct. (b) For those values of k, verify that every member of the family of functions y = A sin(kt) + B cos(kt) is also a solution. y = A sin(kt) + B cos(kt) ⇒ y' = Ak cos(kt) − Bk sin(kt) ⇒ y'' = −Ak2 sin(kt) − Bk2 cos(kt).