7.3 Volumes of Solids with Known Cross Sections
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Transcript 7.3 Volumes of Solids with Known Cross Sections
7.3
VOLUMES WITH KNOWN CROSS
SECTIONS
VOLUMES WITH KNOWN CROSS SECTIONS
A solid has as its base the circle x2 + y2 = 9, and
all cross sections parallel to the y-axis are
squares. Find the volume of the solid.
SOLIDS WITH KNOWN CROSS SECTIONS
If A(x) is the area of a cross section of a solid and A(x)
is continuous on [a, b], then the volume of the solid
from x = a to x = b is
b
V A( x)dx
a
VOLUMES WITH KNOWN CROSS SECTIONS
A solid has as its base the circle x2 + y2 = 9, and
all cross sections parallel to the y-axis are
squares. Find the volume of the solid.
Area of cross section (square)?
3
-3
3
-3
dx
As
2
A (2 y )
2
A 4y
y
2
y-coordinate
So, s = 2y
A 4 9 x
x
2
A 4(9 x )
2
x y 9
2
2
y 9 x
2
2
VOLUMES WITH KNOWN CROSS SECTIONS
A solid has as its base the circle x2 + y2 = 9, and
all cross sections parallel to the y-axis are
squares. Find the volume of the solid.
Area of cross section (square)?
3
-3
y
A 4(9 x )
2
Volume of solid:
x2
V A( x)dx
x1
3
-3
dx
x
3
V 4(9 x 2 ) dx
3
VOLUMES WITH KNOWN CROSS SECTIONS
A solid has as its base the circle x2 + y2 = 9, and
all cross sections parallel to the y-axis are
squares. Find the volume of the solid.
Volume of solid:
3
3
-3
V 4(9 x 2 ) dx
y
3
3
V 4 (9 x 2 ) dx
3
3
1 3
V 49 x x
3 3
3
-3
dx
x
V 418 18
V 436
V 144
KNOWN CROSS SECTIONS
Ex:
The base of a solid is the region enclosed by the
2
y2
ellipse x
1
4 25
The cross sections are perpendicular to the x-axis and
are isosceles right triangles whose hypotenuses are on
the ellipse. Find the volume of the solid.
5
-2
a
a
2
-5
5
1.) Find the area of the cross
section A(x).
-2
a
a
a a (2 y )
2
2
2a 4 y
a 2 y
2
y
2
2
25x 2
A( x ) 25
4
2
-5
1 2
A( x ) a
2
1
A( x )
2 y
2
A( x ) y 2
2
2.) Set up & evaluate the
integral.
25x 2 200 units 3
2 25 4 dx 3
2
EXAMPLE
The base of a solid is the region enclosed by the
triangle whose vertices are (0, 0), (4, 0), and (0,
2). The cross sections are semicircles
perpendicular to the x-axis. Find the volume of
the solid. y
Area of cross section (semicircle)?
1 2
A r
2
2
4
1 1 1
A x 2
2 2 2
x
r is half of the yvalue on the line
y m x b
2
1 1
A x 1
2 4
y
2
1
x2
2
EXAMPLE
The base of a solid is the region enclosed by the
triangle whose vertices are (0, 0), (4, 0), and (0,
2). The cross sections are semicircles
perpendicular to the x-axis. Find the volume of
the solid. y
Area of cross section (semicircle)?
1 1
A x 1
2 4
2
2
Volume
2
1 1
V x 1 dx
2 0 4
4
4
x
(fInt)
V = 2.094