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Vectors P and Q have equal magnitude P and angle between them is Ф.

Magnitude of vector P - Q = SqRoot[ P² + P² - 2 P P Cos Ф ]
        = √2 P SqRoot[ 1 - Cos Ф ] = √2 P SqRoot[ 2 Sin² Ф/2 ] = 2 P SinФ/2