The problem
Suppose $P(x)$ is a polynomial with real coefficients satisfying the condition
$$
P(\cos \theta+\sin \theta)=P(\cos \theta-\sin \theta),
$$
for every real $\theta$. Prove that $P(x)$ can be expressed in the form
$$
P(x)=a_0+a_1\left(1-x^2\right)^2+a_2\left(1-x^2\right)^4+\cdots+a_n\left(1-x^2\right)^{2 n},
$$
for some real numbers $a_0, a_1, a_2, \ldots, a_n$ and nonnegative integer (n).
Hint 1
Using a very standard trigometric identity, we can easily convert the following ,
$$
\begin{aligned}
P(\cos \theta+\sin \theta) & =P(\cos \theta-\sin \theta
\Longrightarrow P\left(\sqrt{2} \sin \left(\frac{\pi}{4}+\theta\right)\right) & =P\left(\sqrt{2} \cos \left(\frac{\pi}{4}+\theta\right)\right) \
\Longrightarrow P(\sqrt{2} \sin x) & =P(\sqrt{2} \cos x)
\end{aligned}
$$
⟹ \(P(\sqrt{2} \sin x)=P(\sqrt{2} \cos x) \quad\)
Assuming,
$\left(\frac{\pi}{4}+\theta\right)=x$ for all reals $x$. So,
$P(-\sqrt{2} \sin (x))=P(\sqrt{2} \sin (-x))=P(\sqrt{2} \cos (-x))=P(\sqrt{2} \cos (x))=P(\sqrt{2} \sin (x))$ for all $x \in \mathbb{R}$. Since $P(x)=P(-x)$ holds for infinitely many $x$, it must hold for all $x$ (since $P(x)$ is a polynomial). so we get that, $P(x)$ is a even polynomial.
Hint 2
$P(\sqrt{2} \cos (x))=P(\sqrt{2} \sin (x))$ implies that
$$
P(t)=P\left(\sqrt{2} \sin \left(\cos ^{-1}(t / \sqrt{2})\right) \text { putting }, x=\cos ^{-1}(t / \sqrt{2})\right.
$$
for infinitely many $t \in[-\sqrt{2}, \sqrt{2}]$.
$$
\sqrt{2} \sin \left(\cos ^{-1}(t / \sqrt{2})\right)=\sqrt{2-t^2} \text { so we get, } P(x)=P\left(\sqrt{2-t^2}\right)
$$
Again as it is a polynomial function we can extend it all $\mathbb{R}$. And we get, $P(x)=P\left(\sqrt{2-x^2}\right)$ for all reals (x)
Hint 3
Since $P(x)$ is even, we can choose an even polynomial $Q(x)$ such that, $Q(x)=P(\sqrt{x+1}) \cdot P(\sqrt{1+x}$=$Q(x)=a_0+a_1 x^2+a_2 x^4+\cdots+a_n x^{2 n}$ now take, $\sqrt{1+x}=y$ and you get the polynomial of required form.
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