Question
$(\sec \theta + \tan \theta ) . ( \sec \theta – \tan \theta ) =$ ?

Answer

$(\sec \theta + \tan \theta )  ( \sec \theta – \tan \theta )$
$= \sec^2\theta – \tan^2\theta ......[(a + b)(a – b) = a^2 – b^2]$
$=1 \cdots \cdots \cdot\left[\begin{array}{l}\because 1+\tan ^2 \theta=\sec ^2 \theta \\ \therefore \sec ^2 \theta-\tan ^2 \theta=1\end{array}\right]$

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