Next, we note in the following figure that the lines from point O to the points s and s+Ds along the trajectory form the same angle Df as is between the vectors and in the vector traingle, and so we can say:

Kinematics of a particle moving on a curve
Next, we note in the following figure that the lines from point O to the points s and s+Ds along the trajectory form the same angle Df as is between the vectors and in the vector traingle, and so we can say:

Hence, we have