- #1
Eclair_de_XII
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Homework Statement
"Suppose that ##F(s) = L[f(t)]## exists for ##s > a ≥ 0##.
(a) Show that if c is a positive constant, then
##L[f(ct)]=\frac{1}{c}F(\frac{s}{c})##
Homework Equations
##L[f(t)]=\int_0^\infty f(t)e^{-st}dt##
The Attempt at a Solution
##L[f(ct)]=\int_0^\infty f(ct)e^{-st}dt##
D: ##e^{-st}## I: ##f(ct)##
D: ##-e^{-st}## I: ##\frac{1}{c}F(ct)##
I: ##F(ct)##
##L[f(ct)]=\int_0^\infty f(ct)e^{-st}dt=(\frac{1}{c}e^{-st}F(ct))+\frac{1}{c}\int_0^\infty e^{-st}F(ct)dt##
D: ##F(ct)## I: ##e^{-st}##
D: ##cf(ct)## I: ##-\frac{1}{s}e^{-st}##
##L[f(ct)]=\int_0^\infty f(t)e^{-st}dt=(\frac{1}{c}e^{-st}F(t))+\frac{1}{c}(-\frac{1}{s}e^{-st}F(ct))+\frac{c}{s}\int_0^\infty e^{-st}f(ct)dt##
##\frac{s-c}{s}\int_0^\infty f(ct)e^{-st}dt=(\frac{1}{c}e^{-st}F(ct))+(-\frac{1}{cs}e^{-st}F(ct))##
##L[f(ct)]=\frac{s}{s-c}(\frac{1}{c}(e^{-st}F(ct)-\frac{1}{s}e^{-st}F(ct))=\frac{s}{s-c}(e^{-st}F(ct))(\frac{1}{c}-\frac{1}{s})##
##L[f(ct)]=\frac{s}{s-c}(e^{-st}F(ct))(\frac{s}{cs}-\frac{c}{cs})=\frac{s}{c}e^{-st}F(ct)##
Can anyone tell me what it is I'm doing wrong?