What is Magnitude of Angle between Vectors?

The vector A = (A_x , A_y) = ( +2.0 m, +2.0 m) and the vector B = (B_x , B_y) = ( +1.0 m, -1.0 m).
What is the magnitude of the angle between the vectors A and B? 

Intermediate calculations (by Google Search):
arccos(((2.05 * 1.05) - (1.95 * 0.95)) / (sqrt(2.05 * 1.95) * sqrt(0.95 * 1.05))) = 81.3594657 degrees
arccos(((1.95 * 0.95) - (2.05 * 1.05)) / (sqrt(2.05 * 1.95) * sqrt(0.95 * 1.05))) = 98.6405343 degrees

Solution: 90 ± 9 degrees