Question

Evaluate each of the following definite integrals by using the Laplace transform table.

Short answer

The value of integral is In 2 .

Step-by-step solution

Given information.

The given expression is ${\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt.$

Laplace transformation

Laplace transformation is a way for solving differential equations. The differential equation in the time domain is first translated into an algebraic equation in the frequency domain. The outcome of solving the algebraic equation in frequency domain is then translated to time domain form to obtain the differential equation's ultimate solution.

By the use of Laplace transformation table$\mathbf{L}\left(\frac{e^{-\mathrm{at}}-e^{-\mathrm{bt}}}{t}\right)\mathbf{=}\mathbf{In}\left(\frac{p+b}{p+a}\right)$

Find the value of given integral equation ∫0∞e-t-e-21tdt.

The given expression is,

${\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt$

From definition of Laplace transforms,

role="math" localid="1659254978863" ${\int }_0^{\infty }{e}^{-pt}\left(\frac{e^{-at}{e}^{-bt}}{t}\right)dt=L\left(\frac{e^{-at}{e}^{-bt}}{t}\right)$

Also

${\int }_0^{\infty }\frac{e^{-t}-e-2t}{t}dt$

It can be written as

${\int }_0^{\infty }{e}^{-pt}\frac{e^{-at}{e}^{-bt}}{t}dt={\int }_0^{\infty }{e}^{-0t}\left(\frac{e^{-at}{e}^{-2t}}{t}\right)dt$

Use this property, and Laplace transform $\left(\frac{e^{-at}-e^{-bt}}{t}\right)$=In $\left(\frac{p+b}{p+a}\right)$(Property (L21) as,

role="math" localid="1659255410649" $$\begin{gathered} {\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt={\int }_0^{\infty }{e}^{-0t}\left(\frac{e^{-t}-e^{-2t}}{t}\right)dt \\ {\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt=L\left(\frac{e^{-t}-e^{-2t}}{t}\right) \\ {\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt=In\left(\frac{p+2}{p+1}\right)......\left(1\right) \end{gathered}$$

Substitute 0 for in equation (1).

$$\begin{gathered} {\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt=In\left(\frac{p+2}{p+1}\right) \\ {\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt=In\left(\frac{p+2}{p+1}\right) \\ =In2 \end{gathered}$$

Hence, the Laplace transforms is${\int }_0^{\infty }\frac{e^{-t}-e^{-2t}}{t}dt=In2$ .