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**I need SERIOUS HELP...integration by parts!!!**

**1. Homework Statement**

Integrate: x^3*e^x

**2. Homework Equations**

**3. The Attempt at a Solution**

I have the answer in my book but I am not understanding why you have to repeat the integration 3 times...

1.) dv = e^x dx

v = e^x

u = x^3

du = 3x^2

S=integral sign lol

S u*dv = uv - S v*du

S x^3*e^3 = x^3*e^x - S 3x^2*e^x dx

And now I have to do this 2 more times but I don't understand why and how I am supposed to know to do that?