Hey,

In order to find the magnitude of a vector the formula is,

R^2= P^2 + Q^2 + 2 PQ cos¢

Now given, R= P+ Q

Squaring both sides,  R^2= P^2 + Q^2 + 2PQ

Equating both the reaction,

R^2= P^2 + Q^2 + 2 PQ cos¢   = R^2= P^2 + Q^2 + 2PQ

Simplifying, cos¢ = 1

= Cos 0° = 1

Therefore the angle is 0 degree.

Hope that helps.

Thank you.