Bentuk a2 - b2

LHS 

\begin{equation*}
\begin{split}
& \cos^4 x \: (1 - \tan^4 x) \\\\
& \cos^4 x \: \left(1 - \dfrac {\sin^ 4}{\cos^4 x}\right) \\\\
& \cos^4 x - \sin^4 x \\\\
& (\cos^2 x)^2 - (\sin^2 x)^2 \quad {\color {blue} a^2 - b^2 = (a + b)(a - b)} \\\\
& (\cos^2 x + \sin^2 x) (\cos^2 x - \sin^2 x) \\\\
& \cos^2 x - \sin^2 x \ce{->} \textbf{RHS}
\end{split}
\end{equation*}

LHS = RHS