Find the Radius From the Area or Circumference
Written by Adi Ackerman, Head Teacher
"Find the Radius From the Area or Circumference" is written for 8th Grade (Ages 13-14). A 7th grader who is already steady on geometry can use it too, and an older student can use it as a short review.
8th grade math worksheet: work backward to the radius from area or circumference, exact for 81π cm² and rounded for 47.1 cm. Answer key included. Part of our area and circumference of circles geometry collection.
How do I use this worksheet?
Begin with the radius and diameter sheets, then switch to exact answers in terms of π. Use Circles Maze or Circle Word Problems Review as a self-checking review.
What students will practice
- Students will find the radius from an area or a circumference given in terms of π.
- Students will divide by 3.14 and take a square root to find a radius from an area.
- Students will round radius answers to the nearest tenth when they are not exact.
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FAQ
How do I use this area and circumference of circles worksheet?
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Begin with the radius and diameter sheets, then switch to exact answers in terms of π. Use Circles Maze or Circle Word Problems Review as a self-checking review.
What does this worksheet teach?
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Work backward to the radius from area or circumference, exact for 81π cm² and rounded for 47.1 cm. The directions on the page read: Find the radius r of each circle and include units. For 1 to 6, the radius is exact. For 7 to 12, use 3.14 for π, round to the nearest tenth and show your work in the box. The page opens with a worked example, and an answer key is included.
Can I use this area and circumference of circles worksheet for homeschool or classroom?
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Yes, both. We made it for 8th grade geometry, and it works the same at home or at school. At home, a student can work through it alone and then go over it with you, which shows quickly what is solid and what needs another look. In class it works as independent practice, homework or a warm-up, and you can print one copy or a full set.
What is the formula for the circumference and area of a circle?
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Circumference is C = 2πr or C = πd, and area is A = πr². For a radius of 8.5 cm, C is about 53.4 cm and A is about 226.9 cm² with 3.14 for π. Area and Circumference From the Radius opens with this example.
What does it mean to leave an answer in terms of π?
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It means you keep the π symbol instead of multiplying by 3.14, so the answer is exact. A circle with a radius of 3.5 cm has a circumference of 7π cm and an area of 12.25π cm². Circle Answers in Terms of Pi and Circle Problems in Terms of Pi practice this.
How do you find the radius from the area of a circle?
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Divide the area by π, then take the square root. For an area of 50.24 m², 50.24 ÷ 3.14 = 16, so the radius is 4 m. Find the Radius From the Area or Circumference has 12 circles to solve this way.
How far does a wheel roll in one turn?
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One full turn rolls a distance equal to the wheel's circumference. A 22 in. wheel turning 40 times rolls 3.14 × 22 × 40 = 2,763.2 in., or about 230.3 ft. Wheels, Tracks and Pizzas starts with this example.
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