From Förberedande kurs i matematik 1
The angle
2+v
makes the same angle with the positive
y
-axis as the angle
v
makes with the positive
x
-axis, and hence we see that the
x
-coordinate for
2+v
is equal to the
y
-coordinate for
v, but with a change of sign, i.e.
cos
2+v
=−sinv=−a
angle
v
angle
2+v
