Guys about the problem on 16:12 , the integrand fuction  is continious for both x = <0  and x>0  so  i assume domain of the integrand fuction is R-0 (there was no other restrictions) but  the undoing trig sub part  (24:21) he assumed theta is <90 so that solution works for x=positives.Did he solve that problem for only x >0 or somehow its related with x<0 too but im missing it ?####[]