Answer: D
\(\csc^2A = 4 - \sec^2A\) Or, \(\csc^2A + \sec^2A = 4\) Or, \(\cfrac{1}{\sin^2A} + \cfrac{1}{\cos^2A} = 4\) Or, \(\cfrac{\cos^2A + \sin^2A}{\sin^2A \cos^2A} = 4\) Or, \(4\sin^2A \cos^2A = 1\) Or, \(2\sin A \cos A = 1\) Or, \(1 - 2\sin A \cos A = 1 - 1 = 0\) Or, \(\sin^2A + \cos^2A - 2\sin A \cos A = 0\) Or, \((\sin A - \cos A)^2 = 0\) Or, \(\sin A - \cos A = 0\) Or, \(\sin A = \cos A = \sin(90^\circ - A)\) Or, \(A = 90^\circ - A\) Or, \(2A = 90^\circ\) Or, \(A = 45^\circ = \cfrac{\pi}{4}\)
\(\csc^2A = 4 - \sec^2A\) Or, \(\csc^2A + \sec^2A = 4\) Or, \(\cfrac{1}{\sin^2A} + \cfrac{1}{\cos^2A} = 4\) Or, \(\cfrac{\cos^2A + \sin^2A}{\sin^2A \cos^2A} = 4\) Or, \(4\sin^2A \cos^2A = 1\) Or, \(2\sin A \cos A = 1\) Or, \(1 - 2\sin A \cos A = 1 - 1 = 0\) Or, \(\sin^2A + \cos^2A - 2\sin A \cos A = 0\) Or, \((\sin A - \cos A)^2 = 0\) Or, \(\sin A - \cos A = 0\) Or, \(\sin A = \cos A = \sin(90^\circ - A)\) Or, \(A = 90^\circ - A\) Or, \(2A = 90^\circ\) Or, \(A = 45^\circ = \cfrac{\pi}{4}\)