Need Help?

  • Notes
  • Comments & Questions

Prove that if $$L^{-1}\{f(s)\}=F(t)$$ then $$L^{-1}\{f(k s)\}=\frac{1}{k} F\left(\frac{t}{k}\right), k>0$$

$$\mathcal{L} L^{-1}\{F(k s)\}=\mathcal {L} \frac{1}{k} F\left(\frac{t}{k}\right)$$

$$f(k s)=\frac{1}{k} \mathcal {L} F(t / k)$$

$$f(s)=\int_{0}^{\infty} e^{-s t} F(t) d t=L[F(t)]$$

$$f\left({ks}\right)=\int_{0}^{\infty} e^{-k s t} F(t) d t$$

put $$u=k t \rightarrow d u=k d t$$

$$=\int_{0}^{\infty} e^{-s u} F\left(\frac{u}{k}\right) \frac{d u}{k}=\frac{1}{k} L\left[F\left(\frac{u}{k}\right)\right]$$

replace u by t

$$f(k s)=\frac{1}{k} L\left[F\left(\frac{t}{k}\right)\right] \quad, k>0$$

Show that if $$F(t)$$ is of class $$A,$$ than $$f^{(n)}(s)=L\left\{(-t)^{n} F(t)\right\}$$
Where $$f(s)=L\{F(t)\}$$

$$f(s)=\mathcal {L}\{F(t)\}=\int_{0}^{\infty} e^{-s t} F(t) d t$$

$$f^{\prime}(s)=\int_{0}^{\infty} -t \cdot e^{-s t} F(t) d t=\mathcal{L}\{-t F(t)\} $$

$$f^{\prime \prime}(S)=\int_{0}^{\infty} t^{2}\cdot e^{-s t} F(t) d t=\mathcal{L}\left\{t^{2} F(t)\right\} $$

$$f^{\prime \prime}(s)=\int_{0}^{\infty}-t^{3} e^{-s t} F(t) d t=L\left\{-t^{3} F(t)\right\} $$

$$f^{n}(s)=\int_{0}^{\infty}(-t)^{n} e^{-s t} F(t) d t=\mathcal{L} \left\{(-t)^n F(t) dt\right\}$$

If $$F(s)=L[f(t)]$$ exists for $$s>a \geq 0$$ and if $$c$$ is a constant
then $$L\left[\mathrm{e}^{c t} f(t)\right]=f(s-c)$$ and $$\mathrm{e}^{c t} f(t)=L^{-1}[F(s-c)]$$

$$\mathcal {L}^{-1} L\left[e^{c t} f(t)\right]=\mathcal {L}^{-1} F(s-c)$$

$$e^{c t} f(t)=L^{-1}[F(s-c)]$$

$$L\left\{e^{c t} f(t)\right\}=\int_{0}^{\infty} {e^{-s t}} {e}^{c t} f(t) d t$$

$$=\int_{0}^{\infty} e^{-(s-c) t} f(t) d t$$

$$L\left\{e^{c t} f(t)\right\}=F(s-c) \quad \quad s>a+c$$

$$\mathcal {L}^{-1} L\left\{e^{c t} f(t)\right\}=\mathcal {L}^{-1}F(s-c)$$

$$e^{c t} f(t)=\mathcal{L}^{-1}[F(s-c)]$$

No comments yet

Join the conversation

Join Notatee Today!