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Thread: Trig Angles tan2x

  1. #1
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    Trig Angles tan2x

    If cosx=-3/5 and pi<x<3pi/2, find tan2x


    I got tan2x=-16/7

    Is my answer right? or is it tan2x=-24/7?
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  2. #2
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    Quote Originally Posted by skeske1234 View Post
    If cosx=-3/5 and pi<x<3pi/2, find tan2x


    I got tan2x=-16/7

    Is my answer right? or is it tan2x=-24/7?
    Hi skeske1234,

    Actually, I got $\displaystyle \tan 2x=\frac{24}{7}$

    If $\displaystyle \cos x =-\frac{3}{5}$, then $\displaystyle x=-3$ and $\displaystyle r=5$ and $\displaystyle y=4$

    So, $\displaystyle \tan x=-\frac{4}{3}$

    $\displaystyle \tan 2x=\frac{2 \tan x}{1-\tan^2x}$

    $\displaystyle \tan 2x=\frac{2(-\frac{4}{3})}{1-(-\frac{4}{3})^2}$

    $\displaystyle \tan 2x = \frac{2(-\frac{4}{3})}{1-\frac{16}{9}}$

    $\displaystyle \tan 2x = \frac{-\frac{8}{3}}{-\frac{7}{9}}$

    $\displaystyle \tan 2x = \frac{24}{7}$
    Last edited by masters; Apr 9th 2009 at 01:07 PM.
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