Trig identities! (1 Viewer)

[professorayton8]

Hi I'm just wondering for trig identities.. How can sin2x turn into 2sinx.cosx? Please help me

 

[Carrotsticks]

Hi I'm just wondering for trig identities.. How can sin2x turn into 2sinx.cosx? Please help me

That IS the given trig identity, or did you want the proof for it?



This is just using the Compound Angle Formulae expansions, but there exist fairly strong geometric proofs using the unit circle and the whole "Angle at centre is twice angle at circumference" theorem.

 

[professorayton8]

Thanks I'm just Wondering.... Are there any tips/cheats that will help me solve trig identities ?

 

[Carrotsticks]

Thanks Im just wondering, besides the tan=sinx/cosx, the sinx+cosx=1 and everything related to it, are there any other trig identities?

I think you meant sin^2 x + cos^2 x = 1.

You should know some basic others like cos(A+B), or sin (A+B), and also cos(2x).

Some more advanced ones are Sums to Products, Products to Sums which isn't exactly in the syllabus, but sometimes a useful tool when it comes to tackling harder problems.

 

[Amundies]

It's not necessary to learn the double angle formulas though. If you know cos(x+y) and sin(x+y), you'll be able to find out cos(2x) and sin(2x). After a while of writing them, you'll just learn them (that's what I did). I assume when you say "everything related to them" to also know what identities are formed when you divide "sin^2 x + cos^2 x = 1" by "cos^2 x" and "sin^2 x" (separately)?

 

[SpiralFlex]

Alternatively you could use a geometric representation of the result. On iPhone ATM can't post any pics until I get back home.