The vector (a+b) should be understood as a whole in drawing. The distance from C vector to it (vector modulus length formula) is always a constant 2. According to the definition of a circle (a set of points whose distance to a point is constant), it can be.

Or, it can be seen that vector C is a combination of vector (a+b) and a vector with a modulus of 2. When drawing with the triangle rule, the endpoint of vector (a+b) is connected with the vector with a modulus of 2 end to end, while the vector with a modulus of 2 can rotate and its trajectory is a circle.