The reference angle for an angle in quadrant 3 is equal to the angle minus 180 degrees.

In this case, the angle is 225 degrees and the reference angle is 225 - 180 = 45 degrees.

The trigonometric functions of 225 degrees are the same as the trigonometric functions of 45 degrees except for the signs.

Sin(225) = -Sin(45) = -.707106781
Cos(225) = -Cos(45) = -.707106781
Tan(135) = Tan(45) = 1

The sine function is negative because the opposite side is below the x-axis.

The cosine function is negative because the adjacent side is to the left of the y-axis.

The tangent function is positive because it is equal to the opposite side divided by the adjacent side and a negative divided by a negative is positive.