Find all values of t where f (t) = t3 − 27t + 5 has horizontal tangent lines. (I.e. find all t so that f′(t) = 0.)

Answer 1

f(t) = t³ - 27t + 5

f'(t) = 3t² - 27 = 0

3(t² - 9) = 0

3(t-3)(t+3) = 0

t = 3 or -3