(iii) Explain why the following function does have an inverse function:

[tex]\[ f(x) = \sin x + \cos x \quad \text{where} \quad -\frac{3 \pi}{4} \leq x \leq \frac{\pi}{4} \][/tex]

(iv) Show that [tex]\( f^{-1}(x) = \arcsin \left(\frac{x}{\sqrt{2}}\right) - \frac{\pi}{4} \)[/tex].