# Plotting the tangent of a curve [duplicate]

I'm new to Mathematica and this is probably child's play for most people, but I wanted to know how to plot the tangent of the function below at the point e=1/4 :

7.523190091795795*^-18 (1.329223358440088*^17 -
3.9876700753202693*^18 e +
Sqrt[-8.944600854152669*^34 +
1.4311361366644287*^37 e^2]), {e, 0, 1}, PlotRange -> {{0.1,
0.3}, {0.1, 0.25}}


Your help would be greatly appreciated!

• Welcome to Mathematica.SE! I suggest the following: 1) As you receive help, try to give it too, by answering questions in your area of expertise. 2) Read the faq! 3) When you see good questions and answers, vote them up by clicking the gray triangles, because the credibility of the system is based on the reputation gained by users sharing their knowledge. Also, please remember to accept the answer, if any, that solves your problem, by clicking the checkmark sign! Jul 2 '15 at 14:21
• Welcome! You should post complete code. At the moment, some parts seem to be missing... this seems like a possible duplicate of mathematica.stackexchange.com/q/18090/131 Jul 2 '15 at 14:21
• There are plenty of answers on the site that seem to address this question: (10669), (18090), (74015); and a bunch of other possibilities. If those answer do not address your question, please indicate how. Jul 2 '15 at 14:22
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This is possibly a duplicate, but the answers given in most cases seem probably more in depth than the OP is looking for, being a newcomer to Mathematica.

First you should define f as a function.

f[e_] := 7.523190091795795*^-18 (1.329223358440088*^17 -
3.9876700753202693*^18 e +
Sqrt[-8.944600854152669*^34 + 1.4311361366644287*^37 e^2]);


From there the tangent line to some point is given by first two terms of the series expansion, so to get the plot you're looking for you can use Mathematica's Series function, which you can read about in the docs.

Plot[{
f[x],
Evaluate@Normal@Series[f[x], {x, 1/4, 1}]
}, {x, 0.1, 0.3}
, PlotRange -> All
]


Here Evaluate@Normal@... is just preparing the output of Series for the Plot function. This give the following image, where the blue line is your original function, and the orange is the tangent: 