Simplify \(\left( \dfrac1{\cos a} - \dfrac{\sin a}{\cos a} \right) \left( \dfrac1{\cos a} + \dfrac{\sin a}{\cos a} \right)\)
I think this will be the same as this rule:
(a+b)(a-b)=a^2-b^2
So, our answer will be
1/cos a minus sin a/cos a
+2094 
