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1. When you have the differential lengths of a cylinder:

dl

_{r}= dr dl

_{[itex]\theta[/itex]}= r d[itex]\theta[/itex] dl

_{z}= dz

Why is dl

_{[itex]\theta[/itex]}equal to r d[itex]\theta[/itex] and not just d[itex]\theta[/itex]?

2. When you have the differential lengths of a sphere:

dl

_{R}= dR dl

_{[itex]\theta[/itex]}= R d[itex]\theta[/itex] dl

_{[itex]\varphi[/itex]}= R sin[itex]\theta[/itex] d[itex]\varphi[/itex]

Why is dl

_{[itex]\theta[/itex]}equal to R d[itex]\theta[/itex] and why is [itex]\varphi[/itex] equal to R sin[itex]\theta[/itex] d[itex]\varphi[/itex]???

I really want to be able to see rather than memorize what each of these differential lengths are equal to.