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 January 6th, 2017, 01:51 PM #11 Member   Joined: Nov 2016 From: Ireland Posts: 72 Thanks: 2 Haha thanks romsek. I'll make you a promise - I've already started going back through complex numbers here. I'll try trig identities again, with your tables in mind and see if it makes any difference. I'll promise to do that, whether it makes any further sense after that... is an entirely different thing - but anyways thanks for the advice bud. Last edited by skipjack; January 6th, 2017 at 03:37 PM.
 January 6th, 2017, 04:01 PM #12 Member   Joined: Nov 2016 From: Ireland Posts: 72 Thanks: 2 So here's what I took from my exploration into complex numbers. Trig substitution is a little like - multiplying out by the conjugate base, in a kind of weird way...?
 January 6th, 2017, 06:29 PM #13 Member   Joined: Oct 2016 From: Melbourne Posts: 77 Thanks: 35 Do you at least have a book of tables with some identities on them? Knowing (or being able to look up) some basic ones is usually enough to be able to prove the rest... Thanks from Kevineamon
 January 6th, 2017, 07:32 PM #14 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,764 Thanks: 1011 Math Focus: Elementary mathematics and beyond Are there any particular identities that are giving you difficulty? Thanks from Kevineamon
 January 7th, 2017, 07:21 AM #15 Member   Joined: Nov 2016 From: Ireland Posts: 72 Thanks: 2 Good shout Prove. I think this might be part of the problem. Now I'm only a lil 1st year uni. 1st semester exams. This is all the trig identities I've been given on the formula sheet. I was trying to calculate things with more advanced trig identities and coming up short - then I just lost patience... Now here's a problem from last year for example. (I'm not sure they had the same formula sheet.) But anyways 1/cosA - cosA = sinA tanA I also know from looking at these things - that 1/cosA = tanA
January 7th, 2017, 07:30 AM   #16
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Quote:
 Originally Posted by Kevineamon I also know from looking at these things - that 1/cosA = tanA
correction ... 1/cotA = tanA

 January 7th, 2017, 07:52 AM #17 Member   Joined: Nov 2016 From: Ireland Posts: 72 Thanks: 2 Lol ahhh well then... See that's what I'm talking about right there, with me trying to do the more complex ones. So you think he only intends to use these identities? Most of these identities are in this form, with a coefficient of 2. Is that to throw you off? Can you simply remove the coefficient and use these trig identities in a more simpler form? What would be the answer to that question up there? Might help me to see the steps. 1/cosA - cosA = sinA tanA
 January 7th, 2017, 08:11 AM #18 Math Team   Joined: Jul 2011 From: Texas Posts: 2,722 Thanks: 1376 $\dfrac{1}{\cos{a}} - \cos{a}$ common denominator is $\cos{a}$ ... $\dfrac{1}{\cos{a}} - \dfrac{\cos^2{a}}{\cos{a}}$ $\dfrac{1-\cos^2{a}}{\cos{a}}$ note $1-\cos^2{a} = \sin^2{a}$ from the Pythagorean identity, $\cos^2{a}+\sin^2{a}=1$ ... $\dfrac{\sin^2{a}}{\cos{a}}$ $\sin{a} \cdot \dfrac{\sin{a}}{\cos{a}}$ $\sin{a} \cdot \tan{a}$ Thanks from Kevineamon
 January 7th, 2017, 08:48 AM #19 Member   Joined: Nov 2016 From: Ireland Posts: 72 Thanks: 2 Ha very good... I mean I followed through it, took me a lil while but I see what you're doing. Coming up with that on my own is quite another thing. Also that last part where SinA/CosB = TanA. I checked that on the calculator, that's right but I mean, it's not there on the formula sheet... Hmmm dunno. I wonder if there is anywhere I could practice, just these types of problems. Seems that's the only way to learn it. Like just these types - not involving the more complex identities. I wonder how I could search for that? If there's a way
January 7th, 2017, 09:06 AM   #20
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Quote:
 Also that last part where SinA/CosB = TanA. I checked that on the calculator, that's right but I mean, it's not there on the formula sheet... Hmmm dunno.
There lies the problem ... you are unfamiliar with the basic trig identities which should be listed in your text.

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