You can't do floating point calculations like this and expect the results to match exactly. You would need to use a tolerance to compare. That being said, instead of this approach maybe you should be looking at the asin( ) function to get one of the values. And then use the periodicity of sin( ) with pencil & paper to figure out the other values relative to the one you found.
Alternatively, you could use a numeric solver to find roots of sin(2*t)-0.4 given various starting points (e.g., from your plot).
What technique/method are you instructed to use for this assignment?
