tan[1/2 arcsin(-7/25)] [Archive]

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jrjack

Aug7-11, 10:29 PM

1. The problem statement, all variables and given/known data

\tan[\frac{1}{2} \arcsin(\frac{-7}{25})]

3. The attempt at a solution

I'm not sure how to take 1/2 the arcsin, should this use the half-angle formula?

Normally I would draw a triangle using the sin value (-7/25), then find the tan value (24/25), but the 1/2 is throwing me off.

How do I start this? Is this 1/2 the sin value (-7/25)= -7/50, then solve for the tan(-7/50)?

Dick

Aug7-11, 10:35 PM

Use the tangent half angle formula. tan(x/2)=??

jrjack

Aug7-11, 10:42 PM

Thanks, so i get

-\sqrt{26}

Does that sound right?

Dick

Aug7-11, 10:55 PM

Thanks, so i get

-\sqrt{26}

Does that sound right?

arcsin(-7/25) is about -0.3. If you take half of that and take the tangent, it's nowhere near -sqrt(26) which is about -5. Is it? You can check these solutions using rough estimates or a calculator.

jrjack

Aug7-11, 10:59 PM

-\sqrt{\frac{1+\cos x}{1-\cos x}}
-\sqrt{\frac{1+\frac{24}{25}}{1-\frac{24}{25}}}
=-\sqrt{26}

Dick

Aug7-11, 11:07 PM

-\sqrt{\frac{1+\cos x}{1-\cos x}}
-\sqrt{\frac{1+\frac{24}{25}}{1-\frac{24}{25}}}
=-\sqrt{26}

tan(0/2)=0. If you put x=0 into your supposed half angle formula, what do you get? Does it work?

jrjack

Aug7-11, 11:14 PM

Sorry, I now realize I have my signs flipped in my formula.
I think my answer should be:-\sqrt{\frac{1}{26}}

Dick

Aug7-11, 11:24 PM

Sorry, I now realize I have my signs flipped in my formula.
I think my answer should be:-\sqrt{\frac{1}{26}}

That doesn't work either because (1-24/25)/(1+24/25) isn't equal to 1/26. Now what's it really equal to??

jrjack

Aug7-11, 11:39 PM

Sorry, I got in a hurry, between typing the tex and working the problem several different ways (wrong of course).

It should equal 1/49, which means my answer should be -\sqrt{\frac{1}{49}}

Dick

Aug7-11, 11:43 PM

Sorry, I got in a hurry, between typing the tex and working the problem several different ways (wrong of course).

It should equal 1/49, which means my answer should be -\sqrt{\frac{1}{49}}

Ok, aside from the fact there is a simpler way to write -1/sqrt(49) could you try and check that using a calculator from your original expression? It's really useful to have a simple way of self-checking whether you are way off or not.

jrjack

Aug7-11, 11:50 PM

Thank you for your help.
I realize I still need to rationalize the denominator, and after checking with my calculator both answers come out to -.142857, so that must be correct.

My final answer should be -1/7

Once again, thank you for your help.

Dick

Aug7-11, 11:54 PM