# Polynomials

## Factorising

#### Quadratics
**Quadratics:** $x^2 + bx + c = (x+m)(x+n)$ where $mn=c$, $m+n=b$  
**Difference of squares:** $a^2 - b^2 = (a - b)(a + b)$  
**Perfect squares:** $a^2 \pm 2ab + b^2 = (a \pm b^2)$  
**Completing the square (monic):** $x^2+bx+c=(x+{b\over2})^2+c-{b^2\over4}$  
**Completing the square (non-monic):** $ax^2+bx+c=a(x-{b\over2a})^2+c-{b^2\over4a}$  
**Quadratic formula:** $x={{-b\pm\sqrt{b^2-4ac}}\over2a}$ where $\Delta=b^2-4ac$ (if $\Delta$ is a perfect square, rational roots)

#### Cubics
**Difference of cubes:** $a^3 - b^3 = (a-b)(a^2 + ab + b^2)$  
**Sum of cubes:** $a^3 + b^3 = (a+b)(a^2 - ab + b^2)$  
**Perfect cubes:** $a^3 \pm 3a^2b + 3ab^2 \pm b^3 = (a \pm b)^3$  

## Linear and quadratic graphs

$$y=mx+c, \quad {x \over a} + {y \over b}=1$$

Parallel lines - $m_1 = m_2$  
Perpendicular lines - $m_1 \times m_2 = -1$


## Cubic graphs

$$y=a(x-b)^3 + c$$

- $m=0$ at *stationary point of inflection*
- in form $y=(x-a)^2(x-b)$, local max at $x=a$, local min at $x=b$
- in form $y=a(x-b)(x-c)(x-d)$: $x$-intercepts at $b, c, d$


## Quartic graphs

$$y=ax^4$$

$$=a(x-b)(x-c)(x-d)(x-e)$$

$$=ax^4+cd^2 (c \ge 0)$$

$$=ax^2(x-b)(x-c)$$

$$=a(x-b)^2(x-c)^2$$

$$=a(x-b)(x-c)^3$$

where
- $x$-intercepts at $x=b,c,d,e$

## Literal equations

Equations with multiple pronumerals. Solutions are expressed in terms of pronumerals (parameters))

## Simultaneous equations (linear)

- Unique solution - lines intersect at point
- Infinitely many solutions - lines are equal
- No solution - lines are parallel

Solving in matrix form - use inverse $A^{-1}= {1 \over {ad-bc}}\begin{bmatrix}d & -b \\ -c & a\end{bmatrix}$. $A^{-1}$ exists for infinite solutions or no solutions ($ad-bc=0$), does not exist for unique solutions ($ad-bc \ne 0$).  
Or use `det` on CAS.