If two vector are equal and rheir resultant is also equal to one of them , then the angle between the two vector is

Explanation

Let `P = ,Q = A, R = A , theta = ?`
`R = sqrt(P^(2)+Q^(2)+2PQ cos theta )`
`A = sqrt(A^(2)+A^(2)+2Axx A cos theta )`
`=sqrt(2A^(2) +2A^(2)cos theta)`
`A^(2) = 2A^(2) (1+cos theta) , cos theta = 1/2 - 1 = -1/2 , theta = 120^(@)`