Using Chebisev polynomials to express sin(nx) & cos(nx) as polinomials of sin(x) and cos(x) $Sin(nx)$ and $cos(nx)$ can be expressed as polynomials of sin(x) and cos(x). I am interested in the way of this expression and a proof (preferably at secondary-school level) as well.

Here's a proof that $\sin(2x)$ can't be written as a polynomial in $\sin x$.
Recall that $\sin(2x) = 2\cos(x)\sin(x)$. Now assume we have a polynomial $p \in \mathbb R[x]$ such that $\sin(2x) = p(\sin(x)).$ Then we also have
$$
2\cos(x)\sin(x) = p(\sin(x)).
$$
Since the l.h.s. is $0$ at $x = 0$, we must have $0 = p(\sin(0)) = p(0)$. This means that $p$ doesn't have a constant term, so we can write $p(x) = xq(x)$ with another polynomial $q \in \mathbb R[x].$ Thus we have
$$
2\cos(x)\sin(x) = \sin(x)q(\sin(x))
$$
for any $x \in \mathbb R$. So we have
$$
2\cos(x) = q(\sin(x)) \quad \forall x \in \mathbb R\setminus\pi\mathbb Z
$$
since $\pi\mathbb Z$ is the zero set of $\sin(x)$. Since both the l.h.s. and the r.h.s of the preceding equation are continuous and $\pi\mathbb Z$ is discrete in $\mathbb R$, we get by continuity
$$
2\cos(x) = q(\sin(x)) \quad \forall x \in \mathbb R.
$$
Now we square this equation and get
$$
4(1-\sin^2(x)) = q(\sin(x))^2 \quad \forall x \in \mathbb R.
$$
Put $r(x) = 4(1-x^2) \in \mathbb R[x]$. With this we can write
$$
r(\sin(x)) = q(\sin(x))^2 \quad \forall x \in \mathbb R.
$$
Since $\sin(x)$ ranges over $[-1,1]$ as $x$ ranges over $\mathbb R$, this is equivalent to
$$
r(x) = q(x)^2 \quad \forall x \in [-1,1].
$$
Now we use the fact that if two polynomials coincide on a non-empty open interval (and $[-1,1]$ contains $(-1,1)$), then these polynomials must be equal. So we get
$$
r(x) = q(x)^2 \quad in\ \mathbb R[x].
$$
But
$$
r(x) = 4(1-x^2) = 4(1-x)(1+x)\quad in\ \mathbb R[x]
$$
is not a square.
So we have reached a contradiction, and the polynomials $q$ and $p$ don't exist.
