<s>[INST] Cannot find angle for trigonometry problem 
A right angle triangle is entrapped within a circle. The triangle entraps within it a circle of its own. The ratio between the big radius and the little radius is $\frac{13}{4}$. What are the angles of the triangle?

Here is the problem as I understand it, given that:
1. $\measuredangle ABC = 90^\circ \rightarrow AC = 2R$
2. The smaller radii are perpendicular  to the triangle.
3. The origin of the smaller circle is the intersection between the angle bisectors of triangle ABC.

Let $\measuredangle ACB = 2\alpha \rightarrow \measuredangle ACG  = \alpha$
$\measuredangle BAC = 90 - 2\alpha \rightarrow \measuredangle GAC = 45 - \alpha$
From here we can construct the following system:
$$\begin{cases} 2R = \frac{r}{\tan \alpha} + \frac{r}{\tan(45 - \alpha)} \\ \frac{R}{r} = \frac{13}{4} \rightarrow R = \frac{13r}{4} \end{cases}$$
$$\downarrow \\ \frac{26r}{4} = r\left(\frac{1}{\tan \alpha} + \frac{1}{\tan(45 - \alpha)}\right) \quad / \div r \\ 6.5 = \frac{\cos \alpha}{\sin \alpha} + \frac{\cos(45 - \alpha)}{\sin(45 - \alpha)}  \quad / {\sin(\alpha \pm \beta) = \sin\alpha\cos\beta \pm \sin\beta\cos\alpha\\ \cos(\alpha \pm \beta) = \cos\alpha\cos\beta \mp \sin\alpha\sin\beta} \\ 6.5 = \frac{\cos \alpha}{\sin \alpha} + \frac{\cos \alpha \cos 45 + \sin \alpha \sin 45}{\sin 45 \ cos \alpha - \sin \alpha \cos 45} \\ 6.5 = \frac{\cos \alpha}{\sin \alpha } + \frac{\frac{\sqrt{2}}{2}}{\frac{\sqrt{2}}{2}} \cdot \frac{\cos \alpha + \sin \alpha}{\cos \alpha - \sin \alpha} \\ 6.5 = \frac{\cos^2 \alpha - \sin\alpha \cos \alpha + \cos \alpha \sin \alpha + \sin^2 \alpha}{\sin(\cos\alpha - \sin \alpha)} \quad / \sin^2\alpha + \cos ^2 \alpha = 1\\ 6.5 = \frac{1}{\sin\alpha (\cos\alpha - \sin \alpha)}$$
From this point on, I have no identity that I can think of which can be applied here to help me solve for $\alpha$. Amusingly enough, I got an answer which was very close to the correct one, but that was only because I made a mistake in the signs of the identity, and the second fracture turned into 1.
I am aware that they may be an identity which can solve this, but I am restricted to only use the identities available here. I have either made a mistake somewhere, or just don't know how to use the available identities to solve the problem.
 [/INST] We have 
$$2R=\frac{r}{\tan(\alpha/2)}+\frac{r}{\tan(\beta/2)},$$
hence 
$$\frac{13}{2}=\frac{1}{\tan(\alpha/2)}+\frac{1}{\tan(\beta/2)}.$$
We know that $\alpha+\beta=\pi/2$, so $\beta/2=\pi/4-\alpha/2$.  Let $t=\tan(\alpha/2)$. From the addition theorems for $\sin$ and $\cos$ derive that 
$$\tan(\pi/4-\alpha/2)=\frac{1-t}{1+t}.$$  Substitute and solve the quadratic equation for $t$.
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