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# How can I draw an arrow from a function of x to the y axis?

Posted 10 years ago
 Given a curve like this with a slider for x: Manipulate[ f[x_] := .5*2 x^2; Plot[{f[x]}, {x, 0, 1}], {{x, .5, "X"}, 0, 1, .01} ]  How can I draw a horizontal arrow from the curve at the current value of x to the y axis with the y value text on the arrow.Thank you. Joe Rimback
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Posted 10 years ago
 Note that your x in your plot needs to be different from your x in your Manipulate slider. So calling the latter xx instead, here is a possibility for you: Manipulate[ Plot[{f[x]}, {x, 0, 1}, Epilog -> {Text[f[xx], {xx/2, f[xx]}, {0, -1}], Arrow[{{xx, f[xx]}, {0, f[xx]}}]}], {{xx, .5, "X"}, 0, 1, .01}, Initialization :> {f[x_] := .5*2 x^2} ] Also note that the way you included your definition of f in your example code is very problematic and leads to a Manipulate that is in an infinite execution loop. Define f in an Initialization option instead as above.
Posted 10 years ago
 Thanks David - I'm learning how to learn. Like your website. I will send you a copy of the annuity app in its current state next week - so you can see how you helped.