Question

URGENT: If θ is a second-quadrant angle and cosθ = -2/3, then tanθ = _____.

Answer 1

In the second quadrant, both cos and tan are negative while only sin is positive.

To find tan, we will use the following property below:

$\large \boxed{ {tan}^{2} \theta = {sec}^{2} \theta - 1}$

Sec is the reciprocal of cos. If cos is a/b then sec is b/a. Since cos is 2/3 then sec is 3/2

$\large{ {tan}^{2} \theta = {( - \frac{3}{2}) }^{2} - 1} \\ \large{ {tan}^{2} \theta = \frac{9}{4} - 1} \\ \large{ {tan}^{2} \theta = \frac{9}{4} - \frac{4}{4} \longrightarrow \frac{5}{4} } \\ \large{tan \theta = \frac{ \sqrt{5} }{ \sqrt{4} } } \\ \large \boxed{tan \theta = \frac{ \sqrt{5} }{2} }$

Since tan is negative in the second quadrant. Hence,

$\large{ \cancel{ tan \theta = \frac{ \sqrt{5} }{2} } \longrightarrow \boxed{tan \theta = - \frac{ \sqrt{5} }{2} }}$

Answer

• tan = -√5/2