# What is the amplitude of y=cos(2/3x) and how does the graph relate to y=cosx?

The amplitude will be the same as the standard $\cos$ function.
Since there is no coefficient (multiplier) in front of the $\cos$, the range will still be from $- 1 \to + 1$, or an amplitude of $1$.
The period will be longer, the $\frac{2}{3}$ slows it down to $\frac{3}{2}$ the time of the standard $\cos$-function.