Find the Laplace transform?

Question

Find the Laplace transform of f(t)=te^at.

Answer: F(s)=1/(s-a)^2, s>a

Answer 1

Lf(t) = ∫_0^∞ t e^te^-stdt = ∫te ^-(s-a)t dt (from 0 to ∞).

Divide and multiply t and dt in the integral by (s-a), introduce new variable z = (s-a)t to come up with the following integral:

                            Lf(t) = (1/(s-a)²) ∫(0,∞) ze^-z dz.

You can make sure that the integral (if integrated in parts) equals 1 (-e^-z with upper limit z=∞ minus -^e-z with z=0). Thus, your answwer is

                                                Lf(t) = 1/(s-a)²