Question

How to plot the graph of `f(x)= 10 cos 3x`?

Answer 1

Your function is a cosine function with amplitude `10` and period `2/3pi`.

Explanation:

The graph of your function is the same as the one from a "normal" cosine BUT:
1] Amplitude will be `10`;
2] Period will be: `period=(2pi)/color(red)(3)` using the `3` in front of the `x` in the argument of your `cos`. This means that your `cos` is "squeezed", completing an entire oscillation between `0` and `2/3pi` instead of between `0` and `2pi`!

Graphically:
`y=f(x)=10cos(3x)`
graph{10(cos(3x)) [-28.88, 28.86, -14.43, 14.45]}

Where you can see the outline of a normal `cos` function but with amplitude of `10` and period `2/3pi=2.09 rad`

I add another plot, easier to "see" (I hope!):