alleryIf $\sin\theta = \dfrac{3}{5}$ and $\theta$ is in the first quadrant, what is $\cos\theta$?
A$\dfrac{4}{5}$
B$\dfrac{5}{4}$
C$\dfrac{3}{4}$
D$\dfrac{5}{3}$
Answer & Solution
Correct answer: A. $\dfrac{4}{5}$
$\cos^{2}\theta = 1 - \sin^{2}\theta = 1 - 9/25 = 16/25 \Rightarrow \cos\theta = \pm 4/5$.
First quadrant → positive → $\cos\theta = 4/5$.
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