The given formula is a sinusoidal equation. Sinusoidal equations are trigonometric functions involving sine and cosine functions. Graphically, they look like wave patterns having amplitudes and periods. The general form of a sinusoidal equation is
y = A cos(Bx + C) + D
where
A = amplitude
B = frequency
C = shift on starting angle
D = shift of wave on the y-axis
Thus, can pattern out that A=1, B=5, C=0 and D=0 in the given equation. The SI unit of frequency is Hertz (Hz) or 1/second. Hence, the frequency of the function is 5 Hz.