Which expression defines the gradient ∇f\nabla f∇f in polar coordinates (r,θ)(r, \theta)(r,θ)?
∂f∂rer+1r∂f∂θeθ\frac{\partial f}{\partial r} \mathbf{e}_r + \frac{1}{r} \frac{\partial f}{\partial \theta} \mathbf{e}_\theta∂r∂fer+r1∂θ∂feθ
∂f∂rer+∂f∂θeθ\frac{\partial f}{\partial r} \mathbf{e}_r + \frac{\partial f}{\partial \theta} \mathbf{e}_\theta∂r∂fer+∂θ∂feθ
∂f∂rer+r∂f∂θeθ\frac{\partial f}{\partial r} \mathbf{e}_r + r \frac{\partial f}{\partial \theta} \mathbf{e}_\theta∂r∂fer+r∂θ∂feθ
1r∂f∂rer+∂f∂θeθ\frac{1}{r} \frac{\partial f}{\partial r} \mathbf{e}_r + \frac{\partial f}{\partial \theta} \mathbf{e}_\thetar1∂r∂fer+∂θ∂feθ