Circumference and Area of Circles 7.9B

Front

What is the formula for the circumference of a circle?

Back

C = 2πr, where C is the circumference and r is the radius.

Front

What is the formula for the area of a circle?

Back

A = πr², where A is the area and r is the radius.

Front

What does 'π' (pi) represent in circle calculations?

Back

π is a mathematical constant approximately equal to 3.14, representing the ratio of the circumference of a circle to its diameter.

Front

If the radius of a circle is 5 cm, what is its circumference?

Back

C = 2π(5) = 10π cm, approximately 31.4 cm.

Front

If the radius of a circle is 3 cm, what is its area?

Back

A = π(3)² = 9π cm², approximately 28.3 cm².

Front

What is the relationship between diameter and radius?

Back

The diameter is twice the radius (D = 2r).

Front

How do you convert diameter to radius?

Back

To find the radius, divide the diameter by 2 (r = D/2).

Front

What is the diameter of a circle with a radius of 4 cm?

Back

D = 2r = 2(4) = 8 cm.

Front

What is the area of a circle with a diameter of 10 cm?

Back

First, find the radius: r = D/2 = 5 cm. Then, A = π(5)² = 25π cm², approximately 78.5 cm².

Front

If a circle has a circumference of 20π cm, what is its radius?

Back

C = 2πr, so r = C/(2π) = 20π/(2π) = 10 cm.