Linked Questions

2
votes
3answers
648 views

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

Possible Duplicate: Why can ALL quadratic equations be solved by the quadratic formula? History of Quadratic Formula How was the quadratic formula $\frac{-b\pm\sqrt{b^2-4ac}}{2a}$ found ...
2
votes
4answers
228 views

How to derive the equation for x in a quadratic equation? [duplicate]

Possible Duplicate: Why can ALL quadratic equations be solved by the quadratic formula? How to derive this: $x = \frac{-b + {\sqrt{b^2 + 4ac}}}{2a}$ From this: $ax^2 + bx + c = 0$ I know ...
0
votes
2answers
64 views

Where does the quadratic formula come from? [duplicate]

Everywhere I look, the $ax^2+bx+c$ portion of the quadratic formula is listed as given. Does anyone know where this comes from? Edit How can we prove that (x+y)^2 = ax^2+bx+c?
15
votes
4answers
8k views

Simple explanation and examples of the Miller-Rabin Primality Test

Coming from an understanding of Fermat's primality test, I'm looking for a clear explanation of the Miller-Rabin primality test. Specifically: I understand that for some reason, having non-trivial ...
12
votes
3answers
935 views

Why are polynomials defined to be “formal”?

Despite the fact that $\forall n, n^3 + 2n \equiv 0 \pmod 3$, I understand that $n^3 + 2n$ (considered as a polynomial with coefficients in $\mathbb Z/3\mathbb Z$) is not equal to the zero polynomial. ...
7
votes
3answers
2k views

Sylvester's determinant identity

Sylvester's determinant identity states that if $A$ and $B$ are matrices of sizes $m\times n$ and $n\times m$, then $$ det(I+AB)=det(I+BA), $$ where in the first case $I$ denotes the $m\times m$ ...
8
votes
2answers
216 views

In generatingfunctionology, for a polynomial $P$ and a differential operator $D$, what does $P(xD)$ mean?

I'm working through some of the exercises in generatingfunctionology. One of the questions is to find the generating function where the $n$th term $a_n=P(n)$ for $P$ a polynomial. The answer is ...
3
votes
5answers
227 views

Proving Quadratic Formula

purplemath.com explains the quadratic formula. I don't understand the third row in the "Derive the Quadratic Formula by solving $ax^2 + bx + c = 0$." section. How does $\dfrac{b}{2a}$ become ...
0
votes
4answers
372 views

How to get from: $a^2 - a + 1 = 0$ to $a = \frac{1}{2}(1\pm\sqrt{1+4})$

Given that $a^2 - a + 1 = 0$, my book says: Therefor $a = \frac{1}{2}(1\pm\sqrt{1+4}).$ I have forgotten all the theory behind this.
9
votes
2answers
641 views

Motivating algebra from quadratic equations

This question gave me pause for thought. We have a quadratic equation $ax^2+bx+c=0$. How much algebra can be motivated from the standard solution. Comments point out that the formula does not apply in ...
4
votes
4answers
291 views

How to find out X in a trinomial

How can I find out what X equals in this? $$x^2 - 2x - 3 = 117$$ How would I get started? I'm truly stuck.
2
votes
2answers
440 views

What are the benefits of using $x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}$ to solve quadratic equations?

When I was in high school, they taught me to solve quadratic equations with this formula: $$x=\frac{\sqrt{4 \text{ac}+b^2}-b}{2 a}$$ EDIT: The original formula is this one: $x = \dfrac{-b \pm ...
4
votes
3answers
174 views

Basic Question Concerning Elementary Functions

I have a homework problem which says If $f(x) = \dfrac{x^2 - x}{x - 1}$ and $g(x) = x$. Is it true that $f = g$? What do they mean by saying is it true that $f = g$? Aren't these two functions ...
3
votes
2answers
1k views

History of Quadratic Formula

My wife is planning a lesson on the quadratic formula for high school students, who have previously learned how to complete the square. It would be nice to open the lesson with some historical ...

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