This applet use another representation to show 1/p+1/q=1/f

You can drag p in the plotting panel and find out the q easily from intersection point at the y-axis.
You can drag and change the focus point,too.

/htdocs/ntnujava/ejsuser/2/users/ntnu/fkh/pqf_pkg/pqf.propertiesFull screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1329kB):double click downloaded file to run it. (65 times by 38 users) , Download EJS source (25 times by 19 users) View EJS source

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