Jawab:

turunan

f'(x) = 0

dari  f(x) = y =  3 cos ( x +  π/4)

titik stationer , y'= 0
- 3 sin (x + π/4) = 0
sin (x + π/4) = 0 = sin 0
x+  π/4= 0 +  k. 2 π  atau x + π/4 = π +  k. 2π
x = -  π/4 + k. 2π  atau x =  3/4 π + k.2π

k = 0 ,  x= - π/4  atau  x=  3/4 π
k = 1  ,  x=  3/4 π  atau x = 2 3/4 π
k = 2 , x = 7/4 π
untuk 0 ≤ x ≤ 2π,  x = { 3/4 π ,  7/4 π)

f(3/4 π) =  3 cos (3/4 π +  π/4)
f(3/4 π) =  3 cos ( π ) =  3(-1) =  -3

f(7/4 π) =  3 cos (7/4π + 1/4π) =
f(7/4 π) =  3 cos (2π) = 3(1) =  3

titik stationer {³/₄ π, -3 }  dan  {⁷/₄π, 3 }