According to the FOURIER THEOREM, periodic sound may be shown to consist of SINE WAVEs in the HARMONIC SERIES, where the Fourier coefficients give the AMPLITUDE and PHASE angle of each component.

Therefore since the trig equation we are solving is sin and it is positive (0.5), then we are in the 1st and 2nd quadrants. We have already found the first solution which is the acute angle from ...