Here's the question you clicked on:
denniel
Prove this identity sin(2A)=2sinAcosA.
Start by considering 2A as A+A ... does that make you think of any particular theorem?
...of course that makes me want to know how to prove the theorem you are thinking of @Tonks
sin(a+a)=sinacosb+cosasinb put the value of a and be here.
We know that sin(2A) = sin(A + A) sin (A + A) = cosA sinA + cosA sinA = 2 sinA cosA QED
@TuringTest try stacking triangles in the unit circle... or is that for cosine?
I know it was some geometric proof like that, I don't remember those darn things...
Is that enough or do you need proof for sin (A + B) = cosA sinB + cosB sinA
formula you have given is wrong @micahwood50
For proof of sin (A + B) = cosA sinB + cosB sinA: http://en.wikipedia.org/wiki/Proofs_of_trigonometric_identities#Sine
http://upload.wikimedia.org/math/3/e/c/3ec0e99aa364deb052280bcd801fcec2.png
yep, that's the one...
@muhammad9t5 It's the same. Your point is?
perhaps you are neglecting the fact that multiplication is commutative muhammad?
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ok i got it!! he has just rearranged the places of cos and sin.
in English that's called the commutative property of addition.
Yeah, sorry about that, though. It's easier for me to remember this formula this way.
@micahwood50 can this destroy the problem of sin(45-30) solve with your formula.
cos45*sin30-cos45sin30
Well, with minus, it's different story.
ok do sin(45+30)=cos45sin30+cos30sin45
Yeah, I got it. Thanks, though.
bro never mind if i disturb you! Accept just like the game of trig!
@muhammad9t5 I know the formulas, you don't have to correct me. Thanks, though.