Polinomu reizināšana
Lai sareizinātu polinomu ar polinomu, katru pirmā polinoma locekli reizina ar katru otra polinoma locekli.
$\begin{array}{l}\left(a+b\right)\cdot \left(m+k\right)=\mathit{am}+\mathit{ak}+\mathit{bm}+\mathit{bk}\\ \\ \left(a+b\right)\cdot \left(m-k\right)=\mathit{am}-\mathit{ak}+\mathit{bm}-\mathit{bk}\\ \\ \left(a-b\right)\cdot \left(m-k\right)=\mathit{am}-\mathit{ak}-\mathit{bm}+\mathit{bk}\end{array}$

Piemērs:
Atver iekavas:
$\left(x-3\right)\left(2x+5\right)=2x\cdot x+5x-3\cdot 2x-3\cdot 5=2{x}^{2}+5x-\mathit{6x}-15=2{x}^{2}-x-15$

Saīsinātās reizināšanas formulas
$\begin{array}{l}{\left(a+b\right)}^{2}={a}^{2}+2\mathit{ab}+{b}^{2}\\ \\ {\left(a-b\right)}^{2}={a}^{2}-2\mathit{ab}+{b}^{2}\\ \\ \left(a-b\right)\cdot \left(a+b\right)={a}^{2}-{b}^{2}\end{array}$

Piemērs:
Atver iekavas
$\begin{array}{l}\left(2-x\right)\left(2+x\right)={2}^{2}-{x}^{2}=4-{x}^{2}\\ \\ {\left(3x-4\right)}^{2}={\left(3x\right)}^{2}-2\cdot 3x\cdot 4+{4}^{2}=9{x}^{2}-24x+16\end{array}$

$\begin{array}{l}{a}^{2}+2\mathit{ab}+{b}^{2}=\left(a+b\right)\left(a+b\right)={\left(a+b\right)}^{2}\\ \\ {a}^{2}-2\mathit{ab}+{b}^{2}=\left(a-b\right)\left(a-b\right)={\left(a-b\right)}^{2}\\ \\ {a}^{2}-{b}^{2}=\left(a-b\right)\left(a+b\right)\end{array}$
$16{y}^{2}-8y+1={4y}^{2}-2\cdot 4y+1={\left(4y-1\right)}^{2}=\left(4y-1\right)\cdot \left(4y-1\right)$