Question

Differentiation

Can you explain to me the step marked with red?

EDIT: http://screencast.com/t/xykWyl0F

Answer 1

... i see nothing marked in red :/

Answer 2

EDIT: http://screencast.com/t/xykWyl0F

Answer 3

that is a trig identity

Answer 4

its best proofed by a picture ....

Answer 5

But what is the identity as an expression?

Answer 6

sin (a+b) = sinb cosa + sina cosb
when a = b

sin (x+x) = sinx cosx + sinx cosx
sin(2x) = 2 sinx cosx


cos(a+b) = cosb cos - sinb sina
when a=b, this has 3 forms that can be played with