1. The problem statement, all variables and given/known data [itex]\int_0^∞ x^2exp(-x/2) dx[/itex] 2. Relevant equations 3. The attempt at a solution Using u substitution: u = x/2 du = 1/2 dx [itex]\int_0^∞ 4u^2exp(-u) du*2[/itex] = 8 [itex]\Gamma(3)[/itex] = 8*3! = 48 But the correct answer is 16 when I plug it in Wolfram's definite integral calculator. I don't see where's my mistake? Thank you very much!