I'm currently having a hard time with this question:

Use differentials (or equivalently, a linear approximation) to approximate

as follows: Let

and find the equation of the tangent line to

at a "nice" point near

. Then use this to approximate

.

I'm familiar with the linear approximation formula, but this is confusing me. I tried getting (with 29.1 as a "nice" point): (cos29.1)(x-29)(sin29.1) but that is incorrect. I think it has something to do with using this number and plugging it into the equation for a tangent line? I'm really not sure. Thanks in advance for any help!