Evaluate the line integral given the path of a helix? Evaluate the line integral 
$$
\int_C  zdx + xdy + xydz
$$
where C is the path of the helix r(t) = (4cost)i + (4sint)j + (t)k on $0\le t \le 2\pi$ 
I solve this problem, but my answer was wrong. 
x= 4cost
y= 4sint
z= t
dx= -4sint dt
dy= 4cost dt
dz= dt
I plugged these into the integral above and integrated to get
$$
4tcost - 4sint + 4t +sin(4t) + 4sint \ |_0^{2\pi}
$$
I solved this and got 16$\pi$
Can someone tell me where I am going wrong?

Your primitive is wrong. We have:
$$
\int_0^{2\pi}(16 \cos^2 t+16 \cos t \sin t-4t \sin t)dt=
$$
$$
= 4\left(2t-\sin t +\sin 2t-2\cos^2 t+t \cos t  \right) |_0^{2\pi}= 24 \pi
$$
