The sequence $ (x_n)$, $ n\in\mathbb{N}^*$ is defined by $ |x_1|<1$, and for all $ n \ge 1$, \[ x_{n\plus{}1} \equal{}\frac{\minus{}x_n \plus{}\sqrt{3\minus{}3x_n^2}}{2}\] (a) Find the necessary and sufficient condition for $ x_1$ so that each $ x_n > 0$.
(b) Is this sequence periodic? And why?
1. **Define a new sequence**: Let us define a new sequence \( \theta_n \) such that \( x_n = \sin(\theta_n) \) and \( \theta_n \in [-90^\circ, 90^\circ] \) for all \( n \in \mathbb{N} \).

2. **Transform the recursion**: Given the recursion
   \[
   x_{n+1} = \frac{-x_n + \sqrt{3 - 3x_n^2}}{2}
   \]
   we substitute \( x_n = \sin(\theta_n) \). Then,
   \[
   x_{n+1} = \sin(\theta_{n+1})
   \]
   and we need to express \( \sin(\theta_{n+1}) \) in terms of \( \theta_n \).

3. **Use trigonometric identities**: Notice that
   \[
   \cos(\theta_n) = \sqrt{1 - \sin^2(\theta_n)} = \sqrt{1 - x_n^2}
   \]
   and the given recursion can be rewritten using trigonometric identities:
   \[
   \sin(\theta_{n+1}) = \frac{\sqrt{3}}{2} \cos(\theta_n) - \frac{1}{2} \sin(\theta_n)
   \]
   Recognize that:
   \[
   \sin(\theta_{n+1}) = \sin(60^\circ) \cos(\theta_n) - \cos(60^\circ) \sin(\theta_n)
   \]
   Using the angle subtraction formula for sine:
   \[
   \sin(\theta_{n+1}) = \sin(60^\circ - \theta_n)
   \]
   Therefore,
   \[
   \theta_{n+1} = 60^\circ - \theta_n
   \]

4. **Determine periodicity**: To check if the sequence is periodic, compute \( \theta_{n+2} \):
   \[
   \theta_{n+2} = 60^\circ - \theta_{n+1} = 60^\circ - (60^\circ - \theta_n) = \theta_n
   \]
   Thus, \( \theta_n \) is periodic with period 2, and consequently, \( x_n = \sin(\theta_n) \) is also periodic with period 2.

5. **Condition for \( x_n > 0 \)**: For \( x_n > 0 \) for all \( n \in \mathbb{N} \), we need \( \sin(\theta_n) > 0 \) for all \( n \). Since \( \theta_n \) alternates between \( \theta_1 \) and \( 60^\circ - \theta_1 \), both must be in the range where sine is positive:
   \[
   \theta_1 \in (0^\circ, 60^\circ)
   \]
   Therefore,
   \[
   x_1 = \sin(\theta_1) \in \left(0, \frac{\sqrt{3}}{2}\right)
   \]

The final answer is \( \boxed{ x_1 \in \left(0, \frac{\sqrt{3}}{2}\right) } \)