At first I was thinking like asinus that these equations were true for all values of beta, but then I realized that can't be because if beta were 90° that would make it 1 - 0 = 0, which is not true. So then I thought about this:
\(\sin \beta - \cos^2 \beta = 0 \\ - \cos^2 \beta = - \sin \beta \\ - \cos^2 \beta + 1 = - \sin \beta + 1 \\ 1 - \cos^2 \beta = - \sin \beta + 1 \\ \sin^2 \beta = - \sin \beta + 1 \\ 0 = - \sin^2 \beta - \sin \beta + 1 \\ 0 = \sin^2 \beta + \sin \beta - 1\)
Then use the quadratic formula or complete the square to find out that
\(\sin \beta = \frac{\sqrt{5}-1}{2}\)
and
\(\sin \beta = \frac{-\sqrt{5}-1}{2}\)
So
\( \beta =\arcsin( \frac{\sqrt{5}-1}{2} )\)
and
\(\beta = \arcsin (\frac{-\sqrt{5}-1}{2})\)
I don't really know if that helps figure out the rest of the problem or not.