Why should L(f(ax-c))=int_0^(oo) e^(-sx)f(ax-c)dx not be L(f(ax−c))=int_0^(oo) e^(-s(ax−c)) f(ax−c)d(ax−c) or equivalently L(f(ax−c))=a int_0^(oo) e^(-s(ax−c)) f(ax−c)dx instead?

Kiana Arias

Kiana Arias

Answered question

2022-09-05

The Laplace transform of a function f=f(x) has the following definition:
(1) L ( f ( x ) ) = 0 e s x f ( x ) d x
However, when f = f ( a x c ), where a and c are arbitrary constants, we have
(2) L ( f ( a x c ) ) = 0 e s x f ( a x c ) d x
with which I got very confused, why should (2) not be
(3) L ( f ( a x c ) ) = 0 e s ( a x c ) f ( a x c ) d ( a x c )
or equivalently
(3) L ( f ( a x c ) ) = a 0 e s ( a x c ) f ( a x c ) d x
instead?

Answer & Explanation

Aleah Harrell

Aleah Harrell

Beginner2022-09-06Added 18 answers

The Laplace transform of a function f(x) is itself not a function of x.
You can write
L ( f ( x ) ) ( s ) = F ( s ) = 0 e s τ f ( τ ) d τ
where τ is a dummy index of integration.
So when you write g ( x ) = f ( a x c ), the Laplace transform G(s) of g(x) is
G ( s ) = 0 e s λ g ( λ ) d λ = 0 e s λ f ( a λ c ) d λ

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