Get the coordinates from a curve?

Questions
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1

Hello, everyone, first time to be here. Beginner of Mathematica.

I wonder how I can get the coordinates of certain points from a curve (may not be able to presented by equations).
Anyway, let’s go with a simple equation:

y==x^2

Now I calculate 4 values (x==1,2,3,4) to get y values. I can have 4 points and then I use listlineplot to get a curve. (I did not draw y==x^2 directly)

So now how can I extract certain point coordinate from this curve. Say I have y==25, how can I get the x value (without using the equation)?

And how can I show the point on the curve with dashed lines extended to x and y directions?

Thank you.

2

For this you can open the DrawingTools palette (from the Graphics menu, not from the Palettes menu) and activate the crosshair tool (to the right in the second row). The rest is explained in the documentation page entiled “How to Get Coordinates for Points in a Plot”.