- #1
betelguese05
- 5
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Surface area of a cone--inconsistency?
Geometry tells us that the surface area of a cone with a circular base is
[tex]SA = \pi rs[/tex]
where s is the slant height of the cone, or
[tex]SA = \pi r \sqrt{r^2 + h^2}[/tex]
Take a cone with a circular base of radius 1 and a height of 4. This formula tells us that
[tex]SA = \pi\sqrt{17}[/tex]
However, trying to approach this problem from a calculus standpoint:
[tex]SA = \int 2 \pi r dh[/tex]
Considering the cone as a triangle for a moment, we have the point (0,4) on the h-axis and (1,0) and the r-axis (where h is the analogue of y and r the analogue of x), and a line connecting them.
This leads to a linear relationship between r and h, such that:
[tex]h = -4r+4[/tex]
or
[tex]r = (0.25)(4-h)[/tex]
Integrating:
[tex]2\pi\int^{4}_{0}(0.25)(4-h)dh[/tex]
=[tex]0.5\pi(4h - 0.5h^2)[/tex]
evaluated from h=0 to 4. However, this yields [tex]4\pi[/tex], not [tex]\pi\sqrt{17}[/tex].
Geometry tells us that the surface area of a cone with a circular base is
[tex]SA = \pi rs[/tex]
where s is the slant height of the cone, or
[tex]SA = \pi r \sqrt{r^2 + h^2}[/tex]
Take a cone with a circular base of radius 1 and a height of 4. This formula tells us that
[tex]SA = \pi\sqrt{17}[/tex]
However, trying to approach this problem from a calculus standpoint:
[tex]SA = \int 2 \pi r dh[/tex]
Considering the cone as a triangle for a moment, we have the point (0,4) on the h-axis and (1,0) and the r-axis (where h is the analogue of y and r the analogue of x), and a line connecting them.
This leads to a linear relationship between r and h, such that:
[tex]h = -4r+4[/tex]
or
[tex]r = (0.25)(4-h)[/tex]
Integrating:
[tex]2\pi\int^{4}_{0}(0.25)(4-h)dh[/tex]
=[tex]0.5\pi(4h - 0.5h^2)[/tex]
evaluated from h=0 to 4. However, this yields [tex]4\pi[/tex], not [tex]\pi\sqrt{17}[/tex].