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# Differentiation problem? watch

1. Hi, I have an equation I need to differentiate, but I'm not sure how;

x=2dtan(θr)cos(θi)

d is constant, θi and θr are angles of incidence and refraction, both varying. The question involves finding a maximum x for an angle θi, so I need to differentiate x with respect to θi.
2. If you only vary , then is constant, so just treat as a constant.
3. (Original post by around)
If you only vary , then is constant, so just treat as a constant.
But varies as varies, so how can you treat is as a constant?
4. (Original post by cameron95)
But varies as varies, so how can you treat is as a constant?
And what rule connects them

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Updated: January 23, 2014
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