y = 2cos2x+2cosx= 2*(2cos^2x-1)+2cosx= 4cos^2x+2cosx-2= 4(cosx+1/4)^2-9/4-1≤cosx≤1-3/4≤cosx+1/4≤5/40≤|cosx+1/4|≤5/40≤4(cosx+1/4)^2≤25/4-9/4≤4(cosx+1/4)^2-9/4≤16/4