Tan

i have this code that i wrote and its supposed make the arrow point to the mouse :

onClipEvent(enterFrame){
dx=_root._xmouse-this._x
dy=_root._ymouse-this._y
reverse=Math.tan(dy/dx)
inverse=Math.atan(reverse)
this._rotation=inverse
}

is it because atan is not the inverse of tan or did i do something else wrong