How do use udu substitution to integrate

 If you mean ∫ (e^x)(e^3x)dx then: 

Just multiply e^x and e ^3x to get e^4x .. 

Let u=4x. Then du = 4dx, so dx = 1/4du .. 

You have ∫e^4xdx = ∫1/4e^udu = 1/4e^u = 1/4e^4x + c.

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