A particle with mass $m$ travels with speed $v_0$ and angle $\theta$ compared to the horizontal ground. Air resistance given as $F_R = -m \alpha v$, where $v$ is the particle's speed, is working against the projectile.
The particle's motion is described as
$$\frac{{dv_x} }{dt} + \alpha v_x = 0$$
$$\frac{{dv_y} }{dt} + \alpha v_y = -g$$
where
$$v_x = \frac{{dx} }{dt}$$
$$v_y = \frac{{dy} }{dt}$$
Show that the particles position is given as
$$ x(t) = \frac{{1} }{\alpha}(v_0 \cos \theta)(1-e^{-\alpha t})$$ $$ y(t) = \frac{{1} }{\alpha}(v_0 \sin \theta + \frac{g}{\alpha})(1-e^{-\alpha t}) - \frac{g}{\alpha}t$$
I understand how to set up the equations and do the integrals (at least a part of it) and get the following result:
$$ x(t) = \frac{{1} }{\alpha}(1-e^{-\alpha t})$$
What I don't understand is where the $v_0 cos \theta$ is coming from. I'm thinking it has something to do with decomposing forces in $x$ and $y$ direction, but I can't seem to work it out and where it goes in the equation.
(I haven't started with $y(t)$ yet).