y = 1/4cosx - 1
-1 <= cosx <= 1
-1/4 <= 1/4cosx <= 1/4
-1/4 - 1 <= 1/4cosx - 1 <= 1/4 - 1
-1.25 <= 1/4cosx - 1 <= -0.75
ответ. [-1.25; -0.75]
y = 1/4cosx - 1
-1 <= cosx <= 1
-1/4 <= 1/4cosx <= 1/4
-1/4 - 1 <= 1/4cosx - 1 <= 1/4 - 1
-1.25 <= 1/4cosx - 1 <= -0.75
ответ. [-1.25; -0.75]