Unit vectors are defined in the radially outward direction r : r ^ ^ = ∂ ∂ r ∂ ∂ r = ( cos ⁡ ⁡ θ θ , sin ⁡ ⁡ θ θ ) and in the direction at right angles to r : θ θ ^ ^ = ∂ ∂ 2 r ∂ ∂ r ∂ ∂ θ θ = ( − − sin ⁡ ⁡ θ θ , cos ⁡ ⁡ θ θ ) .