it is given that R= vector P + vector Q
let new resultat be R’
so when Q is doubled then R perpendicular P.
solution: vector P + vector Q = Vector R
as R’ is perpendicular to P
there dot product wil be zero
(vector P) . (vector 2Q)= 0
so cos theta= vector P+vector2Q/2Q
theta = cos-1P+ 2Q/2Q