Expressions, equations & relationships · TEKS 7.5B, 7.8C, 7.9B
Circles & π
π (about 3.14 or 22/7) is the ratio of any circle’s circumference to its diameter.
Key steps
- Circumference: C = πd or C = 2πr.
- Area: A = πr².
- Radius = diameter ÷ 2.
- Diameter from circumference: d = C ÷ π.
- Semicircle: half the circle’s area.
Worked example
Diameter 10 ft. Area? (π = 3.14)
- r = 5.
- A = 3.14 × 5² = 3.14 × 25 = 78.5.
- Answer: 78.5 ft²
Watch out: Area needs the RADIUS. Using the diameter (3.14 × 10²) gives 314, four times too big.
Practice
5 questions with new numbers every set. Get 4 of 5 to mark this topic as practiced.