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I found a solution online which it said : "It's easy noted that $AG.AE$ = $AD^2$ = $AF^2$ (Using tangent of circumscribed circle)"

I found this not obvious at all. I know that $AD = AF$ but why it had to equal to the product of two inline line?

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  • $\begingroup$ do you want proof of this therom?but proof is also simple just using pythgoreous theorem in different right angle triangle.And that line AE called as secant line $\endgroup$ Jun 22, 2013 at 13:00
  • $\begingroup$ Wow, thanks a lot for the keyword. I found a Wikipedia entry which I will use to answer my own question. Thanks. $\endgroup$
    – 5argon
    Jun 22, 2013 at 13:19

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Nevermind, I've found it! It's called "secant-tangent theorem", "intersecting chords theorem", or the "power-of-a-point theorem". Which you can learn in this link..

http://en.wikipedia.org/wiki/Power_of_a_point

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  • $\begingroup$ to proove it takes just two right angle triangle $\endgroup$ Jun 22, 2013 at 13:23

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