$\phi = (\omega t - {\beta _2}) - (\omega t - {\beta _1}) = ({\beta _1} - {\beta _2})$
Resultant amplitude $A = \sqrt {A_1^2 + A_2^2 + 2{A_1}{A_2}\cos ({\beta _1} - {\beta _2})} $

($A$) $v_P+v_R=2 v_Q$
($B$) The rate of change in beat frequency is maximum when the car passes through $Q$
($C$) The plot below represents schematically the variation of beat frequency with time
(image)
($D$) The plot below represents schematically the variation of beat frequency with time
(image)