Now , Let's another an example of Trigonometric graph. `y = cos(x)`

`X`	`-2π`	`-(3π)/2`	`-π`	`-π/2`	`0`	`π/2`	`π`	`(3π)/2`	`2π`
`Y`	`1`	`0`	`-1`	`0`	`1`	`0`	`-1`	`0`	`1`

You can see that the cos curve starts at 1, decrease to 0, further decrease to -1. than it increase back to 1 Compare with sin graph, it is periodic too. it looks just shift a little bit.