# Multiplication On The Parabola

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## Theorem

Let $\map f x = x^2$.

Let $A = \tuple {x_a, y_a}$ and $B = \tuple {x_b, y_b}$ be points on the curve $\map f x$ so that $x_a \le x_b$.

Then the line segment joining $A B$ will cross the $y$-axis at $-x_a x_b$.

## Proof

Let $\map f x = x^2$.

Then:

$\map f {x_a} = x_a^2$

and:

$\map f {x_b} = x_b^2$

Then the slope of the line segment joining $A B$ will be:

 $\displaystyle m$ $=$ $\displaystyle \frac {x_a^2 - x_b^2} {x_b - x_a}$ $\displaystyle$ $=$ $\displaystyle \frac {\paren {x_a - x_b} \paren {x_b + x_a} } {x_b - x_a}$ Difference of Two Squares $\displaystyle$ $=$ $\displaystyle x_b + x_a$ cancelling

The result follows from taking the equation of the line defined by its slope and either point $A$ or $B$ to calculate the $y$-intercept.

Proceeding with point $A$:

 $\displaystyle x_a^2$ $=$ $\displaystyle \paren {x_b + x_a} x_a + b$ $\displaystyle x_a^2 - \paren {x_a + x_b} x_a$ $=$ $\displaystyle b$ $\displaystyle x_a^2 - x_a^2 - x_b x_a$ $=$ $\displaystyle b$ $\displaystyle -x_b x_a$ $=$ $\displaystyle b$

$\blacksquare$