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Sagot :
To determine which formulas can correctly find the surface area of a right circular cylinder where [tex]\( h \)[/tex] is the height, [tex]\( r \)[/tex] is the radius, [tex]\( L A \)[/tex] is the lateral area, and [tex]\( B A \)[/tex] is the base area, we need to examine the relationships and formula derivations for each term.
The surface area [tex]\( A \)[/tex] of a right circular cylinder is given by this formula:
[tex]\[ A = 2\pi r(h + r) \][/tex]
To break this down:
1. Lateral Area (LA): This represents the area of the side surface and is calculated as:
[tex]\[ L A = 2\pi rh \][/tex]
2. Base Area (BA): This represents the area of one of the circular bases. Since a right circular cylinder has two bases, the total base area is twice the area of one base:
[tex]\[ B A = \pi r^2 \][/tex]
So, for two bases, it is:
[tex]\[ 2 \cdot B A = 2 \pi r^2 \][/tex]
Combining these, the total surface area [tex]\( A \)[/tex] can be written as:
[tex]\[ A = 2\pi rh + 2\pi r^2 \][/tex]
Now, let's check the given formulas:
A. [tex]\( B A + 2\pi r^2 \)[/tex]
- This formula incorrectly adds [tex]\( \pi r^2 \)[/tex] for one base and doubles it again, which does not account for the lateral area.
B. [tex]\( \pi r^2 + \pi rh \)[/tex]
- This formula incorrectly finds the surface area by not doubling the base area, and it also incorrectly multiplies the lateral area term by [tex]\( \pi r \)[/tex] rather than [tex]\( 2 \pi r \)[/tex].
C. [tex]\( 2\pi r^2 + 2\pi rh \)[/tex]
- Correct! This matches the exact formula for the surface area derived above.
D. [tex]\( B A + L A \)[/tex]
- If we assign [tex]\( B A = \pi r^2 \)[/tex] and [tex]\( L A = 2\pi rh \)[/tex], we get:
[tex]\[ B A + L A = \pi r^2 + 2\pi rh \][/tex]
However, the phrasing strongly suggests the intention of doubling the base areas:
[tex]\[ \text{If doubled: } 2 \pi r^2 + 2 \pi r h \][/tex]
Therefore, it is also correct if we consider the total base areas.
E. [tex]\( L A + (\pi r^2) \)[/tex]
- [tex]\( L A = 2\pi rh \)[/tex] and adding one base area only gives [tex]\( \pi r^2 \)[/tex] instead of doubling it, which doesn't fully match the original formula unless it was intended to implicitly consider both bases.
Given the correct understanding of surface area calculations, the correct formulas are options C, D, and E (the latter if given considerations).
Thus, the formulas that correctly find the surface area of a right cylinder are:
[tex]\[ \boxed{3, 4, 5} \][/tex]
The surface area [tex]\( A \)[/tex] of a right circular cylinder is given by this formula:
[tex]\[ A = 2\pi r(h + r) \][/tex]
To break this down:
1. Lateral Area (LA): This represents the area of the side surface and is calculated as:
[tex]\[ L A = 2\pi rh \][/tex]
2. Base Area (BA): This represents the area of one of the circular bases. Since a right circular cylinder has two bases, the total base area is twice the area of one base:
[tex]\[ B A = \pi r^2 \][/tex]
So, for two bases, it is:
[tex]\[ 2 \cdot B A = 2 \pi r^2 \][/tex]
Combining these, the total surface area [tex]\( A \)[/tex] can be written as:
[tex]\[ A = 2\pi rh + 2\pi r^2 \][/tex]
Now, let's check the given formulas:
A. [tex]\( B A + 2\pi r^2 \)[/tex]
- This formula incorrectly adds [tex]\( \pi r^2 \)[/tex] for one base and doubles it again, which does not account for the lateral area.
B. [tex]\( \pi r^2 + \pi rh \)[/tex]
- This formula incorrectly finds the surface area by not doubling the base area, and it also incorrectly multiplies the lateral area term by [tex]\( \pi r \)[/tex] rather than [tex]\( 2 \pi r \)[/tex].
C. [tex]\( 2\pi r^2 + 2\pi rh \)[/tex]
- Correct! This matches the exact formula for the surface area derived above.
D. [tex]\( B A + L A \)[/tex]
- If we assign [tex]\( B A = \pi r^2 \)[/tex] and [tex]\( L A = 2\pi rh \)[/tex], we get:
[tex]\[ B A + L A = \pi r^2 + 2\pi rh \][/tex]
However, the phrasing strongly suggests the intention of doubling the base areas:
[tex]\[ \text{If doubled: } 2 \pi r^2 + 2 \pi r h \][/tex]
Therefore, it is also correct if we consider the total base areas.
E. [tex]\( L A + (\pi r^2) \)[/tex]
- [tex]\( L A = 2\pi rh \)[/tex] and adding one base area only gives [tex]\( \pi r^2 \)[/tex] instead of doubling it, which doesn't fully match the original formula unless it was intended to implicitly consider both bases.
Given the correct understanding of surface area calculations, the correct formulas are options C, D, and E (the latter if given considerations).
Thus, the formulas that correctly find the surface area of a right cylinder are:
[tex]\[ \boxed{3, 4, 5} \][/tex]
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