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If the infinite curve y = e^−2x, x ≥ 0, is rotated about the x-axis. Find the area of the resulting surface.

Sagot :

The area of the resulting surface of this infinite curve is π units.

In this question,

The infinite curve is y = e^−2x, x ≥ 0.

The curve is rotated about x-axis.

Since x ≥ 0, the limits will be 0 to ∞.

Then the area of the resulting surface is,

[tex]A= 2\pi \lim_{b\to \infty} (\int\limits^\infty_0{e^{-2x} } \, dx )[/tex]

Now substitute,

u = -2x

⇒ du = -2dx

⇒ dx = [tex]-\frac{1}{2} du[/tex]

Then,

[tex]\int\limits{-\frac{1}{2}e^{u} } \, du =-\frac{1}{2}\int\limits{e^{u} } \, du[/tex]

Now substitute u and du, we get

⇒ [tex]-\frac{1}{2} \int\limits {e^{-2x} }(-2) \, dx[/tex]

⇒ [tex]-\frac{-2}{2} \int\limits {e^{-2x} } \, dx[/tex]

⇒ [tex](1) \int\limits {e^{-2x} } \, dx[/tex]

⇒ [tex]\int\limits {e^{-2x} } \, dx[/tex]

Thus the area of the resulting surface is

[tex]A= 2\pi \int\limits^\infty_0{e^{-2x} } \, dx[/tex]

⇒ [tex]A= 2\pi [{e^{-2x}(\frac{1}{-2} ) } \,]\limits^\infty_0[/tex]

⇒ [tex]A= \frac{2\pi}{-2} [{e^{-2(\infty)}-e^{-2(0)} } \,]\\[/tex]

⇒ [tex]A= -\pi [0-1} \,]\\[/tex]

⇒ [tex]A= \pi[/tex]

Hence we can conclude that the area of the resulting surface is π units.

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