Expressions, Equations, and Relationships>Determining the Circumference and Area of Circles(TEKS.Math.7.9.B)
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Texas 7th Grade Math › Expressions, Equations, and Relationships>Determining the Circumference and Area of Circles(TEKS.Math.7.9.B)
A circle has a radius of 7.5 cm. What is the circumference?
47.10 cm
23.55 cm
176.63 square centimeters
94.20 cm
Explanation
Use $C=2\pi r$ because the radius is given. $C=2(3.14)(7.5)=47.1\text{ cm}$ (about $47.10\text{ cm}$). Circumference uses linear units (cm), not square units.
A circular pool has a diameter of 24 feet. What is the area?
75.36 feet
1808.64 square feet
452.16 square feet
37.68 square feet
Explanation
Area uses $A=\pi r^2$. With diameter 24 ft, $r=12$ ft. $A=3.14(12)^2=3.14(144)=452.16\text{ square feet}$. Use square units for area.
A circular sticker has a diameter of 9 inches. What is the circumference?
56.52 inches
14.13 inches
63.59 square inches
28.26 inches
Explanation
For circumference with diameter, use $C=\pi d$. $C=3.14(9)=28.26\text{ inches}$. Circumference is a length (linear units), not square inches.
A circular garden has a radius of 3.2 meters. What is the area?
20.10 meters
32.15 square meters
128.61 square meters
10.05 square meters
Explanation
Use $A=\pi r^2$ because the radius is given. $A=3.14(3.2)^2=3.14(10.24)=32.15\text{ square meters}$. Area must be in square units.
A bike wheel has a radius of 14 inches. What is the circumference?
87.92 inches
43.96 inches
175.84 inches
87.92 square inches
Explanation
Use $C=2\pi r$ for circumference with radius. $C=2(3.14)(14)=87.92\text{ inches}$. Circumference is linear, so do not use square units.