1. The problem statement, all variables and given/known data [itex]∫\frac{4dx}{\sqrt{-4x - x^2}}[/itex] 2. Relevant equations [itex]arcsin(\frac{x}{a}) = \frac{1}{\sqrt{a^2-X^2}}[/itex] Correct Answer: 4sin^{-1}[itex]\frac{1}{2}[/itex](x + 2) + c 3. The attempt at a solution I am completely lost with this one. I tried pulling a [itex]\sqrt{x}[/itex] out of the bottom. [itex]∫\frac{4dx}{\sqrt{x}\sqrt{-4 - x}}[/itex] u = [itex]\sqrt{x}[/itex] du = [itex]\frac{1}{2}x^{-1/2}dx[/itex] [itex]8∫\frac{du}{\sqrt{-4-u^2}}[/itex] So [itex]a^2 = -4[/itex] [itex]\sqrt{-4}[/itex] is not a real number. I would appreciate any help you could give. Thanks.