Answer
$1$
Work Step by Step
$\tan^2{\theta}+1 = \sec^2{\theta}$
By rearranging the equation:
$$\sec^2{\theta}-\tan^2{\theta}=1$$
$\therefore \sec^2{18^{\circ}}-\tan^2{18^{\circ}}=\boxed{1}$
Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)
by Sullivan III, Michael
Published by
Pearson
ISBN 10:
0-32193-104-1
ISBN 13:
978-0-32193-104-7
Chapter 5 - Trigonometric Functions - Section 5.3 Properties of the Trigonometric Functions - 5.3 Assess Your Understanding - Page 418: 78
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