Double angle and half angle formulae help!

[dianzhang]

i dont get what to do
same with cos(2x)cos(pie/3) + sin(2x)sin(pie/3) = [sprt(3)]/2
and another question,
if sin(theta) = -3/7 and pie<theta<3pie/2 find sin(theta/2)
and another question,
simplify cos(4x)sin(4x)
i am so lost.

[TrueTears]

4tanx = 6(2tanx/(1-tan^2x))

let u = tanx

4u = 6(2u/(1-u^2)

u = 0

solve tanx = 0

[dianzhang]

4tanx = 6(2tanx/(1-tan^2x))

let u = tanx

4u = 6(2u/(1-u^2)

u = 0

solve tanx = 0

how did you get to u=0

[Special At Specialist]

4u = 8u / (1 - u^2)
4u(1 - u^2) = 8u
4u - 4u^3 - 8u = 0
-4u^3 - 4u = 0
u^3 + u = 0
u(u^2 + 1) = 0
u = 0 or u^2 = -1 --- NOT REAL
u = 0 is only real solution.