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How can I assign $a,b,c,d$ values $\pm \tan\theta,\pm{1\over\tan\theta}$ so that ${(a-b)(c-d)\over (a-d)(b-c)}=\tan^2(2\theta)$? Thank you for helping.

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    I'm not exactly sure how this works. If you assign values $\alpha \tan \theta$, $\beta \tan \theta ...$ to $a, b, c, d$ respectively, where $\alpha, \beta ...$ correspond to one of $\pm 2, \pm \frac{1}{2}$ then you can cancel out the $\tan \theta$s and you'll be left with a constant.2012-02-08
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    @Sp3000: actually the other tan related factor is a reciprocal, also I think the 2's are unnecessary... I have edited the question, hope it makes more sense now.2012-02-08

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Since , $\tan^2 2\theta =\frac{4\tan^2\theta}{(1-\tan^2\theta)^2}$ solution is :

$a=\frac{-1}{\tan \theta}$ , $b=\frac{1}{\tan \theta}$ , $c=\tan \theta$ , $d=-\tan \theta$

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    Oh it does check out (and the 2s really were unnecessary). Out of curiosity may I ask how you got that answer or did you just try every possibility?2012-02-08
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    @Sp3000,I had a moment of lucidity :-)2012-02-08
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    Thanks, pedja. I have removed the 2's in the question, is it possible to get an answer if the 2's are still in? I am asking this because They are actually in the question on my homework sheet...2012-02-08
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    Personally i don't think it is possible if the 2's are in because you cant get the bottom bit of the fraction as $1-\tan^2\theta$ there must be a factor of 4 in front , right?2012-02-08
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    I meant a factor of 4 in front of $\tan\theta$. I am unable to find a combination to get $(\tan^2(2\theta))/2$ would you mind shedding some light?2012-02-08
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    @DominicD,just multiply by $2$ all variables...2012-02-08
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    I am a bit confused. Originally the options were $\pm 2\tan\theta,\pm {1\over 2\tan\theta}$...2012-02-08
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    @DominicD,In that case I don't think that is possible to express $\tan^2 2\theta$...2012-02-08