# Why Manipulate command is not working when ploting the tangent line?

Posted 10 years ago
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 Hi all,I'm trying to get an interactive output that allows me to plot both the function f and its tangent line y by changing the parameter x0. However I'm not able to obtain the plot of y. This is the code:f[x_] = x^2;y[x_] = f[x0] + f'[x0]*(x - x0);Manipulate[Plot[{y[x], f[x]}, {x, -4, 10}], {x0, 0, 6}]  And this what I get:As you see no tangent line appears.Thanks a lot!
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Posted 10 years ago
 Thanks, Bruce!
Posted 10 years ago
 Notes on typing posts, including keeping code looking good, are in http://community.wolfram.com/groups/-/m/t/151347I put your code into MCode boxes and the function arguments reappeared.
Posted 10 years ago
 Your original posted code does not work as in the illustration.  The function  f  and  y  are missing their arguments.The reason it wouldn't work with  f  and  y  is that  Manipulate  localizes  x0.  This means the  x0  symbol in the definition of  y  is a different symbol than the  x0  in  Manipulate.Here's a way to get what seems intended:f[x_] = x^2;y[x_, x0_] = f[x0] + f'[x0]*(x - x0);Manipulate[Plot[{y[x, x0], f[x]}, {x, -4, 10}], {x0, 0, 6}]
Posted 10 years ago
 Thanks a lot Michael now it works!I did write the missing arguments of y and f in Mathematica but for some reason they dissapeared when copying them here.
Posted 10 years ago
 I found that if I write the whole expression of the tangent line inside the Manipulate command it works:f[x_] = x^2;Manipulate[Plot[{f[x0] + f'[x0]*(x - x0), f[x]}, {x, -4, 10}], {x0, 0, 6}] Why is that?Thanks!
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