Eliminate $\theta$ from $\lambda\cos2\theta=\cos(\theta + \alpha) \space$ and $\space \space\lambda \sin2\theta=2\sin(\theta + \alpha)$ Eliminate $\theta$ from $\lambda \cos2\theta=\cos(\theta + \alpha)$ and $\lambda\sin2\theta=2\sin(\theta + \alpha)$
My approach:
Dividing the RHS and LHS of both equations by $\lambda$, then squaring and adding them, we get,
$$\frac{\cos^2(\theta+\alpha)}{\lambda^2}+\frac{4\sin^2(\theta+\alpha)}{\lambda^2}=\cos^22\theta + \sin^22\theta=1$$
$$\Rightarrow \sin^2(\theta+\alpha)=\frac{\lambda^2-1}{3}$$
I am unable to proceed.
 A: From $\tan^3\theta=\tan\alpha$
$\dfrac{\sin\theta}{(\sin\alpha)^{1/3}}=\dfrac{\cos\theta}{(\cos\alpha)^{1/3}}=\pm\dfrac1{\sqrt{(\cos\alpha)^{2/3}+(\sin\alpha)^{2/3}}}$
From $\lambda\cos2\theta=\cos(\theta+\alpha)$
$$\lambda(\cos^2\theta-\sin^2\theta)=\cos\theta\cos\alpha-\sin\theta\sin\alpha$$
$$\implies\lambda\dfrac{(\cos\alpha)^{2/3}-(\sin\alpha)^{2/3}}{(\cos\alpha)^{2/3}+(\sin\alpha)^{2/3}}=\pm\dfrac{(\cos\alpha)^{1/3+1}-(\sin\alpha)^{1/3+1}}{\sqrt{(\cos\alpha)^{2/3}+(\sin\alpha)^{2/3}}}$$
Assuming $\cos\alpha\ne\sin\alpha,$
$$\lambda=\pm\left((\cos\alpha)^{2/3}+(\sin\alpha)^{2/3}\right)^{1+1-1/2}$$
$$\implies\lambda^{2/3}=(\cos\alpha)^{2/3}+(\sin\alpha)^{2/3}$$
A: Take the square root and then apply $\arcsin$ (odd function) to both sides of the equation. You will obtain
$$\theta  = \pm \arcsin \sqrt{\dfrac{\lambda^2-1}{3}}$$
$$\theta  = - \alpha \pm \arcsin \sqrt{\dfrac{\lambda^2-1}{3}}$$
Note, that by taking the square root of $\lambda^2-1$ we restrict the possible values of $\lambda$. We have $|\lambda|\geq 1$.
A: Result
First we calculate $\theta$
$$\theta = \frac{1}{2} \arcsin\left(\frac{4}{3}(1-\frac{1}{\lambda^2})\right)\tag{1}$$
if $\lambda^2 >1$, and else no solution.
This can be combined with the equation derived in the OP
$$\sin^2(\theta+\alpha)=\frac{\lambda^2-1}{3}\tag{2}$$
to solve for $\alpha$ so that both quantities are eliminated and the equations are completely solved in terms of $\lambda$.
Derivation of (1)
We have
$$ \cos(\theta+\alpha)=\lambda \cos(2 \theta)$$
$$ \sin(\theta+\alpha)=\frac{1}{2}\lambda \sin(2 \theta)$$
so that
$$1 = \lambda ^2 \cos(2 \theta)^2 + \frac{1}{4} \lambda ^2 \sin(2 \theta)^2$$
which eliminates $\alpha$.
Hence, observing $\cos(2 \theta)^2 + \sin(2 \theta)^2 = 1$, follows $(1)$.
A: We have
\begin{align*}
\tan2\theta&=2\tan(\theta+\alpha)\\
\frac{2\tan\theta}{1-\tan^2\theta}&=\frac{2(\tan\theta+\tan\alpha)}{1-\tan\theta\tan\alpha}\\
\tan\theta-\tan^2\theta\tan\alpha&=\tan\theta(1-\tan^2\theta)+\tan\alpha(1-\tan^2\theta)\\
\tan^3\theta&=\tan\alpha
\end{align*}
From $\lambda\sin2\theta=2\sin(\theta+\alpha)$,
\begin{align*}
2\lambda\sin\theta\cos\theta&=2(\sin\theta\cos\alpha+\cos\theta\sin\alpha)\\
\lambda&=\frac{\cos\alpha}{\cos\theta}+\frac{\sin\alpha}{\sin\theta}\\
&=\frac{\cos\alpha}{\cos\theta}\left(1+\frac{\tan\alpha}{\tan\theta}\right)\\
\lambda^2&=\frac{\sec^2\theta}{\sec^2\alpha}\left(1+\frac{\tan\alpha}{\tan\theta}\right)^2\\
&=\left(\frac{1+\tan^2\theta}{1+\tan^2\alpha}\right)(1+\tan^2\theta)^2\\
&=\frac{(1+\tan^2\theta)^3}{1+\tan^6\theta}\\
&=\frac{1+2\tan^2\theta+\tan^4\theta}{1-\tan^2\theta+\tan^4\theta}\\
(\lambda^2-1)(1+\tan^4\theta)&=(\lambda^2+2)\tan^2\theta\\
\tan^2\theta+\frac{1}{\tan^2\theta}&=\frac{\lambda^2+2}{\lambda^2-1}
\end{align*}
Note that
$$\tan^2\alpha+\frac{1}{\tan^2\alpha}=\tan^6\theta+\frac{1}{\tan^6\theta}=\left(\tan^2\theta+\frac{1}{\tan^2\theta}\right)^3-3\left(\tan^2\theta+\frac{1}{\tan^2\theta}\right)$$
Therefore, $\displaystyle \tan^2\alpha+\frac{1}{\tan^2\alpha}=\left(\frac{\lambda^2+2}{\lambda^2-1}\right)^3-3\left(\frac{\lambda^2+2}{\lambda^2-1}\right)$.
A: I'm late here and @lab bhattacharjee has already mentioned the beautiful answer to this question. I am only writing this to add another way to reach that answer.

Expanding $\cos(\theta+\alpha)$ and $\sin(\theta+\alpha)$, our equations can be written as, $$\lambda\cos2\theta=\cos\theta\cos\alpha-\sin\theta\sin\alpha$$
$$\lambda\sin2\theta=2\sin\theta\cos\alpha+2\cos\theta\sin\alpha$$
Now we solve for $\cos\alpha$, $$\lambda(2\cos\theta\cos2\theta+\sin\theta\sin2\theta)=2\cos\alpha(\cos^2\theta+\sin^2\theta)$$
$$\lambda(\cos\theta\cos2\theta+\cos\theta)=2\cos\alpha$$
$$\lambda\cos\theta(\underbrace{\cos2\theta+1}_{2\cos^2\theta})=2\cos\alpha$$
$$\lambda\cos^3\theta=\cos\alpha\tag1$$
Similarly, solving for $\sin\alpha$, we get, $$\lambda\sin^3\theta=\sin\alpha\tag2$$
Finally, using the famous identity $\cos^2\theta+\sin^2\theta=1$,
$$\lambda^{2/3}=(\cos\alpha)^{2/3}+(\sin\alpha)^{2/3}$$
$\blacksquare$
