Given that $\sin\theta = \dfrac{3}{5}$ and $\theta$ is obtuse, find $\tan\theta$.
A$\dfrac{3}{4}$
B$-\dfrac{3}{4}$CORRECT
C$-\dfrac{4}{3}$
D$\dfrac{4}{3}$
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Why the answer is B, and why the others tempt you.
From $\sin^2\theta + \cos^2\theta = 1$, $\cos\theta = \pm\dfrac{4}{5}$. Because $\theta$ is obtuse (second quadrant), cosine is negative, so $\cos\theta = -\dfrac{4}{5}$ and $\tan\theta = \dfrac{\sin\theta}{\cos\theta} = -\dfrac{3}{4}$. Ignoring the quadrant gives $\dfrac{3}{4}$, and inverting gives $-\dfrac{4}{3}$.
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