# Derivative of $$$r \cos{\left(\theta \right)}$$$ with respect to $$$r$$$

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### Your Input

**Find $$$\frac{d}{dr} \left(r \cos{\left(\theta \right)}\right)$$$.**

### Solution

**Apply the constant multiple rule $$$\frac{d}{dr} \left(c f{\left(r \right)}\right) = c \frac{d}{dr} \left(f{\left(r \right)}\right)$$$ with $$$c = \cos{\left(\theta \right)}$$$ and $$$f{\left(r \right)} = r$$$:**

**Apply the power rule $$$\frac{d}{dr} \left(r^{n}\right) = n r^{n - 1}$$$ with $$$n = 1$$$, in other words, $$$\frac{d}{dr} \left(r\right) = 1$$$:**

Thus, $$$\frac{d}{dr} \left(r \cos{\left(\theta \right)}\right) = \cos{\left(\theta \right)}$$$.

### Answer

**$$$\frac{d}{dr} \left(r \cos{\left(\theta \right)}\right) = \cos{\left(\theta \right)}$$$A**