The reference angle for an angle in quadrant 4 is equal to 360 degrees minus the angle.
In this case, the angle is 315 degrees and the reference angle is 360 - 315 = 45 degrees.
The trigonometric functions of 315 degrees are the same as the trigonometric functions of 45 degrees except for the signs.
Sin(315) = -Sin(45) = -.707106781
Cos(315) = Cos(45) = .707106781
Tan(315) = -Tan(45) = -1
The sine function is negative because the opposite side is below the x-axis.
The cosine function is positive because the adjacent side is to the right of the y-axis.
The tangent function is negative because it is equal to the opposite side divided by the adjacent side and a negative divided by a positive is negative.