I feel like I am missing something really obvious, but I don't understand
why the point (3, 2π/5) is not on the graph of r = 3cos(5θ) .
If you plug in 2π/5 for θ, you get r = 3cos( 5(2π/5) ) = 3cos( 2π ) = 3(1) = 3
So why isn't (3, 2π/5) a point on the graph?
Okay....it looks like it is interpreting (3, 2π/5) as Cartesian coordinates, not as polar coordinates.