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관련코스 다변수미적분학
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선행 키워드
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### Definition

$\nabla f = \cfrac{1}{h_i}{\partial f \over \partial q^i} \mathbf{e}^i$
If we have a 3-dimensional function $f(x,y,z)$, the derivative will have to give us the relationship between $df$ and $d\vec{r} = (dx,dy,dz)$.
This kind of derivative is called gradient.
As we know $df$ can be represented as below using partial derivatives:
$df = \left(\frac{\partial f}{\partial x}\right)dx + \left(\frac{\partial f}{\partial y}\right)dy + \left(\frac{\partial f}{\partial z}\right)dz$
So, in Cartesian coordinates, we can define gradient as follows
$\nabla f \equiv \left( \frac{\partial f}{\partial x}, \frac{\partial f}{\partial y}, \frac{\partial f}{\partial z} \right)$
so we can write $df = \nabla f \cdot d\vec{r}$

#### Cylindrical coordinates

$\nabla f={\partial f \over \partial \rho}\hat{\boldsymbol \rho} + {1 \over \rho}{\partial f \over \partial \varphi}\hat{\boldsymbol \varphi} + {\partial f \over \partial z}\hat{\mathbf z}$

#### Spherical coordinates

$\nabla f={\partial f \over \partial r}\hat{\mathbf r}+ {1 \over r}{\partial f \over \partial \theta}\hat{\boldsymbol \theta}+ {1 \over r\sin\theta}{\partial f \over \partial \varphi}\hat{\boldsymbol \varphi}$