Given that f (x) = x, G (x) = RF (x) + SiNx is a decreasing function on the interval [- 1,1] Finding the maximum of R

Given that f (x) = x, G (x) = RF (x) + SiNx is a decreasing function on the interval [- 1,1] Finding the maximum of R

g'(x)=r+cosx
x∈[-1,1],cosx∈[cos1,1],r+cosx∈[r+cos1,r+1]
G '(x) is a decreasing function on [- 1,1], that is, G' (x) is constant on [- 1,1], that is, R + cosx ≤ 0
So as long as R + cosx takes the maximum value ≤ 0
If R + 1 is less than or equal to 0, the maximum value of R is - 1