Given vectors u = (-2, 3) and v = (-6, 9), what is the measure of the angle between u and v?
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Let r be a radius of a given circle and α be an angle, that corresponds to a sector.
The circle area is
and denote the sector area as 
.
Then
(the ratio between area is the same as the ratio between coresponding angles).
.
The circle area is
Then