If $\tan A = \dfrac{1}{2}$ and $\tan B = \dfrac{1}{3}$, find the value of $\tan(A + B)$.
A$\dfrac{5}{6}$
B$\dfrac{5}{7}$
C$1$CORRECT
D$\dfrac{1}{6}$
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Why the answer is C, and why the others tempt you.
$\tan(A+B) = \dfrac{\tan A + \tan B}{1 - \tan A\tan B} = \dfrac{\frac{1}{2} + \frac{1}{3}}{1 - \frac{1}{6}} = \dfrac{\frac{5}{6}}{\frac{5}{6}} = 1$. Adding the tangents without dividing gives $\dfrac{5}{6}$, and using a plus sign in the denominator gives $\dfrac{5}{7}$.
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