**If theta = (-1pi)/4, then **
sin (theta) =
cos (theta) =
From trigonometry we know that 1pi/4 = 45 degrees, so a negative value from 0 would make it 315 degrees (Think about the full unit circle).
Using the definition of references angles in relation to the 4th quadrant, the reference angle is equal to 360 - x = 360 - 315 = 45 degrees.
The sine and cosine of 45 degrees are both equal to the following fraction:
(square root of 2)/(2).
However, because it is in the 4th quadrant, sine of 315 is negative, i.e. -(square root of 2)/(2).
cosine(315) = (square root of 2)/(2).
sine(315) = -(square root of 2)/(2).