First, what did you get for the derivative of y with respect to x?
I know the basic deffinition of the differential, I just don't know how to apply it in the case of a trigonometric function
I tried to solve it, f'(2) = (1/cos^2(4(2)+7))(4) =>
= (1/cos^2(15))(4) = ... and I recognize I have no idea how to deal with the trig expression cos^2(15)), do I have to consider it as cos(225), which gives me -0,707106, and I just have to plug in the values... (sorry, I haven't done any math since 2002, and I revised a bit of basic trigonometry, but I'm still lost in the trig identities and calculations. I hope though, I would manage to improve that by the time of my final exam...)