Question

Prove this identity sin(2A)=2sinAcosA.

Answer 1

Start by considering 2A as A+A ... does that make you think of any particular theorem?

Answer 2

...of course that makes me want to know how to prove the theorem you are thinking of @Tonks

Answer 3

sin(a+a)=sinacosb+cosasinb put the value of a and be here.

Answer 4

We know that sin(2A) = sin(A + A)

sin (A + A) = cosA sinA + cosA sinA = 2 sinA cosA

QED

Answer 5

@TuringTest try stacking triangles in the unit circle... or is that for cosine?

Answer 6

I know it was some geometric proof like that, I don't remember those darn things...

Answer 7

Is that enough or do you need proof for sin (A + B) = cosA sinB + cosB sinA

Answer 8

http://youtu.be/BlEtOG4NRsg

Answer 9

formula you have given is wrong @micahwood50

Answer 10

it's correct

Answer 11

For proof of sin (A + B) = cosA sinB + cosB sinA: http://en.wikipedia.org/wiki/Proofs_of_trigonometric_identities#Sine

Answer 12

http://upload.wikimedia.org/math/3/e/c/3ec0e99aa364deb052280bcd801fcec2.png

Answer 13

yep, that's the one...

Answer 14

@muhammad9t5 It's the same. Your point is?

Answer 15

perhaps you are neglecting the fact that multiplication is commutative muhammad?