September 26th, 2013, 07:37 AM  #1 
Newbie Joined: Sep 2013 Posts: 1 Thanks: 0  Showing limit equals derivative
This is giving me so much trouble and I have no idea why. I just have to show that f'(x) = ( f(x + e)  f(x  e) ) / 2e for x = 5 and e = .001 The actual function is f(x) = e^(sinx) + xln(3 + cosx) and I'm almost positive the derivative is f'(x) = cos(x)e^(sinx) + ln(3 + cosx)  xsinx/(3 + cosx) When I evaluate the derivative at 5 I get 2.363208918 but when I evaluate the function with the small e I get 1.402409569 I just can't figure out why these numbers don't match up. Thanks. 
September 26th, 2013, 10:45 AM  #2 
Senior Member Joined: Jul 2012 From: DFW Area Posts: 633 Thanks: 94 Math Focus: Electrical Engineering Applications  Re: Showing limit equals derivative
Hi jpanderson8, and welcome to the forums. I think that the derivative is correct, however, I think you are making the calculation using degrees. If you use radians, you should get 2.75783 (for both calculations). Also, I know that it is difficult without using but just to make sure let me restate: . 

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derivative, equals, limit, showing 
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