Computer sales are generally subject to seasonal fluctuations. An analysis of the sales of a computer manufacturer during 2008–2010 is approximated by the function

where t represents time in quarters (t = 1 represents the end of the first quarter of 2008), and s(t) represents computer sales (quarterly revenue) in millions of dollars. Use a double-angle identity to express s(t) in terms of the cosine function.

Computer sales are generally subject to seasonal fluctuations. An analysis of the sales of a computer manufacturer during 20082010 is approximated by the function s(t) = 0.098 cos't + 0.387 11 12