Question

Multiply (in horizontal and vertical arrangement):

(10) (a + 4b) (a − 4b)

Answer 1

(a + 4b) (a − 4b)
Horizontal arrangement:
= (a + 4b) × a − (a + 4b) × 4b
= a × a + 4b × a − a × 4b − 4b × 4b
= a² + 4ba − 4ab − 16b²
= a² − 16b²

Vertical arrangement:
$\begin{array}{c}\begin{array}{cccccc}& & & a& +& 4b\end{array}\\ \begin{array}{cccccc}& & \times & a& -& 4b\end{array}\\ \overline{\begin{array}{cccccc}& & & -4ab& -& 16b^2\end{array}}\\ \begin{array}{cccccc}+& a^2& +& 4ab& & \end{array}\\ \underline{\overline{\begin{array}{cccccc}& a^2& +& 0ab& -& 16b^2\end{array}}}\end{array}$