This is probably a very obvious question, but I am going through the problems listed here: http://users.cms.caltech.edu/~vidick/teaching/120_qcrypto/HW1_solutions.pdf

  and, I don't understand in 1b) why it says that its obvious that <orth(Psi_1)|Psi_2>| would equal sin(theta) given that |<Psi_1|Psi_2>| = cos(theta)

This is probably a very obvious question, but I am going through this problem set and I don't understand why in 1b) it says that it is obvious that $|\langle\psi_1^\perp|\psi_2\rangle|=\sin\theta$ given that $|\langle\psi_1|\psi_2\rangle| = \cos\theta$.