Youngkwon C. answered 01/21/16
Tutor
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Knowledgeable and patient tutor with a Ph.D. in Electrical Eng.
Hi Zach,
Recall that sinθ is negative and tanθ is positive
in the third quadrant of x-y axis,
in which cosθ is negative.
According to the rule sin2θ + cos2θ = 1
or the Pythagorean theorem,
we can get cosθ = -12/13.
Then, from tanθ = sinθ/cosθ,
we get tanθ = 5/12.