How was the quadratic formula found and proven? [duplicate]

How was the quadratic formula $\frac{-b\pm\sqrt{b^2-4ac}}{2a}$ found and proven?

marked as duplicate by Rahul, Nate Eldredge, M Turgeon, Pedro Tamaroff♦, Rudy the ReindeerSep 3 '12 at 18:18

• Do you mean historically or how to prove it? – vanguard2k Sep 3 '12 at 9:48
• Previously: History of quadratic formula (seems to be answered in the comments), and Why can ALL quadratic equations be solved by the quadratic formula? – Rahul Sep 3 '12 at 9:51
• ah, thanks for the edit. – Lucas - Better Coding Academy Sep 3 '12 at 9:55
• For future reference, when you are adding tags to your question, please read the short descriptions that pop up and make sure they are appropriate. For example, the description for proof-theory says "Proof theory is an area of logic that studies proof as formal mathematical objects," which is not what your question is about. – Rahul Sep 3 '12 at 10:04

• I don't think that is true. al-Khwarizmi didn't have a concept of negative numbers, so he treated equations of the form $ax^2 + bx = c$, $ax^2 + c = bx$, and $ax^2 = bx + c$ as separate cases. – MJD Sep 3 '12 at 11:16
by completing the square. $a\neq 0$, $ax^2+bx+c=a\left(x^2+\dfrac{b}{a}x+\dfrac{c}{a}\right)=\cdots$