Convexity of $S=\left\{ \frac{\underline{x}^HA\underline{x}}{\underline{x}^H\underline{x}} , \underline{x} \in \mathbb C ^{n}\right\}$

Question

i've to prove that the following set is convex:

Let $A \in \mathbb{C}^{n \times n}$

$$S=\left\{ \frac{\underline{x}^HA\underline{x}}{\underline{x}^H\underline{x}} , \underline{x} \in \mathbb C ^n\right\}$$

i've already tried to use the definition without success and then i tried to express every $x \in S$ as a convex combination of some elements, without success too.

Can you help me?