Question:
Which of the following is incorrect?
(a) $\sin \theta=-\frac{1}{5}$
(b) $\cos \theta=1$
(c) $\sec \theta=\frac{1}{2}$
(d) $\tan \theta=20$
Solution:
If $\sec \theta=\frac{1}{2}$
$\Rightarrow \cos \theta=2$
i. e $\theta=\cos ^{-1}(2)$
i.e No solution
$\therefore \sec \theta=\frac{1}{2}$ is incorrect
Hence, the correct answer is option C.
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