what were your answers?
i got @ = 45, 135, 225, 315
1 - 2 cos²@ = 0
- 2cos²@ = -1
cos²@ = 1/2
cos@ = ±1/√2
your answers will also depend on the range. e.g. 0 ≤ @ ≤ 360
hope this helps, i actually have an exam on trig tomorrow so that's what i've been doing..