Find the range of function f (x) = SiNx sinxcosx + cosx, X ∈ (π / 2,3 π / 2)

Find the range of function f (x) = SiNx sinxcosx + cosx, X ∈ (π / 2,3 π / 2)

Because sinxcosx = [(SiNx + cosx) ^ 2-1] / 2, let y = SiNx + cosx, because x ∈ (π / 2,3 π / 2) so y ∈ [- √ 2,1), so f (x) = SiNx sinxcosx + cosx can be changed to f (y) = y - (y ^ 2-1) / 2 = - 1 / 2 (Y-1) ^ 2 + 1, y ∈ [- √ 2,1)