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## Homework Statement

r = r(t)

[tex]\theta[/tex] = [tex]\theta[/tex](t)

x = r cos([tex]\theta[/tex])

dx/dt =dr/dt cos([tex]\theta[/tex]) - r sin([tex]\theta[/tex]) d[tex]\theta[/tex]/dt

## The Attempt at a Solution

Where does the d[tex]\theta[/tex]/dt come from at the end of the derivative? I know I'm using product rule here because r and theta are both functions of t. But, the derivative of cos is just -sin. Why would there be a d[tex]\theta[/tex]/dt at the end?