Was there mention of "amplitude-phase form" in the class? There is information on this in the differential equations class in this PDF: http://ocw.mit.edu/courses/mathematics/18-03sc-differential-equations-fall-2011/unit-i-first-order-differential-equations/sinusoidal-functions/MIT18_03SCF11_s7_3text.pdf That will allow you to convert to amplitude phase form with a cosine. To change that to a sine, use this: $\cos(x\pm \pi/2) = \mp sin(x).$