[tex]\bf \textit{difference of squares}
\\ \quad \\
(a-b)(a+b) = a^2-b^2\qquad \qquad
a^2-b^2 = (a-b)(a+b)
\\\\\\
\textit{and recall that }i^2=-1\\\\
-------------------------------\\\\
y^4-16\implies (y^2)^2-4^2\implies (y^2-4)(y^2+4)\\\\\\ (y^2-2^2)(y^2+4)
\implies
(y-2)(y+2)(y^2+4)\quad
\begin{cases}
+4=-(-2^2)\\
\qquad -(-1\cdot 2^2)\\
\qquad -(i^2\cdot 2^2)\\
\qquad -(2^2i^2)\\
\qquad -(2i)^2
\end{cases}
\\\\\\
(y-2)(y+2)[y^2-(2i)^2]\implies (y-2)(y+2)[(y-2i)(y+2i)]
\\\\\\
(y-2)(y+2)(y-2i)(y+2i)[/tex]
