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September 26th, 2013, 07:37 AM   #1
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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.
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September 26th, 2013, 10:45 AM   #2
jks
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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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