Identify the maximum and minimum values of the function y = 10 cos x in the interval [-2π, 2π].

We know that the maximum and minimum values of the function y = cos x in the interval [-2π, 2π] are 1 and -1 respectively.
 
The graph of y = 8 cos x is vertical stretch by a factor of 8 units. Therefore the graph y=cos x, = the maximum and minimum values of the function y = cos x in the interval [-2π, 2π]. So they are 8 and -8 respectively.  

max = 8,  ∈[−2π,2π]
                                                    = Solution 
min = −8.  ∈[−2π,2π]

The first one minimumi hope my answer you.