Berk's question via email about an antiderivative

In summary, the integral $\displaystyle \int{ \mathrm{e}^{-2\,x}\cos{(3\,x)}\,\mathrm{d}x}$ can be evaluated using Integration By Parts twice or by solving the associated homogeneous differential equation and using the principle of superposition. The resulting solution is $\displaystyle \frac{3}{13}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} - \frac{2}{13}\,\mathrm{e}^{-2\,x} \cos{ \left( 3\,x \right) } + C$.
  • #1
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Evaluate $\displaystyle \begin{align*} \int{ \mathrm{e}^{-2\,x}\cos{(3\,x)}\,\mathrm{d}x} \end{align*}$

This requires using Integration By Parts twice...

$\displaystyle \begin{align*} I &= \int{\mathrm{e}^{-2\,x}\cos{ \left( 3\,x \right) } \,\mathrm{d}x} \\ I &= \frac{1}{3}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} - \int{ -\frac{2}{3}\,\mathrm{e}^{-2\,x} \sin{(3\,x)}\,\mathrm{d}x } \\ I &= \frac{1}{3}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} + \frac{2}{3} \int{ \mathrm{e}^{-2\,x} \sin{(3\,x)} \,\mathrm{d}x } \\ I &= \frac{1}{3}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} + \frac{2}{3} \left[ -\frac{1}{3}\,\mathrm{e}^{-2\,x} \cos{(3\,x)} - \int{ \frac{2}{3}\,\mathrm{e}^{-2\,x} \cos{ \left( 3\,x \right) } \,\mathrm{d}x } \right] \\ I &= \frac{1}{3}\,\mathrm{e}^{-2\,x}\sin{ \left( 3\,x \right) } - \frac{2}{9}\,\mathrm{e}^{-2\,x} \cos{(3\,x)} - \frac{4}{9} \int{ \mathrm{e}^{-2\,x} \cos{(3\,x)} \,\mathrm{d}x } \\ I &= \frac{1}{3}\,\mathrm{e}^{-2\,x} \sin{ \left( 3\,x \right) } - \frac{2}{9} \,\mathrm{e}^{-2\,x} \cos{ \left( 3\,x \right) } - \frac{4}{9}\,I \\ \frac{13}{9}\,I &= \frac{3}{9}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} - \frac{2}{9}\,\mathrm{e}^{-2\,x} \cos{ \left( 3\, x\right) } \\ I &= \frac{3}{13}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} - \frac{2}{13}\,\mathrm{e}^{-2\,x} \cos{ \left( 3\,x \right) } \end{align*}$

Thus $\displaystyle \begin{align*} \int{ \mathrm{e}^{-2\,x} \cos{ \left( 3\,x \right) } \,\mathrm{d}x} = \frac{3}{13}\,\mathrm{e}^{-2\,x} \sin{(3\,x)} - \frac{2}{13}\,\mathrm{e}^{-2\,x} \cos{ \left( 3\,x \right) } + C \end{align*}$
 
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  • #2
Another approach would be to consider:

\(\displaystyle y=\int e^{ax}\cos(bx)\,dx\)

Thus:

\(\displaystyle \d{y}{x}=e^{ax}\cos(bx)\)

The homogeneous solution is:

\(\displaystyle y_h(x)=c_1\)

And the particular solution will take the form:

\(\displaystyle y_p(x)=e^{ax}\left(A\cos(bx)+B\sin(bx)\right)\)

Hence:

\(\displaystyle y_p'(x)=e^{ax}\left((Aa+Bb)\cos(bx)+(Ba-Ab)\sin(bx)\right)\)

Substituting into the ODE, we obtain:

\(\displaystyle e^{ax}\left((Aa+Bb)\cos(bx)+(Ba-Ab)\sin(bx)\right)=e^{ax}\cos(bx)\)

\(\displaystyle (Aa+Bb)\cos(bx)+(Ba-Ab)\sin(bx)=\cos(bx)+0\sin(bx)\)

Equating coefficients, we obtain the system:

\(\displaystyle Aa+Bb=1\)

\(\displaystyle Ba-Ab=0\)

Solving this system, we obtain:

\(\displaystyle (A,B)=\left(\frac{a}{a^2+b^2},\frac{b}{a^2+b^2}\right)\)

And so our particular solution is:

\(\displaystyle y_p(x)=\frac{e^{ax}}{a^2+b^2}\left(a\cos(bx)+b\sin(bx)\right)\)

And then by the principle of superposition, there results:

\(\displaystyle y(x)=y_h(x)+y_p(x)=c_1+\frac{e^{ax}}{a^2+b^2}\left(a\cos(bx)+b\sin(bx)\right)\)

And so we conclude:

\(\displaystyle \int e^{ax}\cos(bx)\,dx=\frac{e^{ax}}{a^2+b^2}\left(a\cos(bx)+b\sin(bx)\right)+C\)

In the given problem, we have:

\(\displaystyle a=-2,\,b=3\)

Plugging those in, we have:

\(\displaystyle \int e^{-2x}\cos(3x)\,dx=\frac{e^{-2x}}{13}\left(3\sin(bx)-2\cos(bx)\right)+C\)
 

FAQ: Berk's question via email about an antiderivative

What is an antiderivative?

An antiderivative is the inverse of a derivative. It is a function that, when differentiated, gives the original function that was differentiated. In simpler terms, it is a function that "undoes" the process of differentiation.

Why is an antiderivative important?

Antiderivatives are important in calculus because they allow us to find the area under a curve, which is essential in many real-world applications such as calculating volumes, work, and distance traveled.

How do you find an antiderivative?

To find an antiderivative, you can use a set of rules known as the "antiderivative rules" or "integration rules." These rules involve reversing the process of differentiation, such as taking the exponent down and increasing the power by one.

Can all functions have an antiderivative?

No, not all functions have an antiderivative. Some functions, such as those that involve transcendental functions like sine and cosine, do not have closed-form antiderivatives and can only be approximated.

What is the relationship between derivatives and antiderivatives?

The relationship between derivatives and antiderivatives is that they are inverse operations of each other. The derivative of a function gives its rate of change, while the antiderivative gives the original function. In other words, the derivative and antiderivative "undo" each other.

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