Area & Circumference of Circles Flashcards

All 6 cards in this deck

What is the formula for the circumference of a circle?

$C = 2\pi r = \pi d$

What is the formula for the area of a circle?

$A = \pi r^2$

True or false? The area of a circle of diameter $10$ cm is $\pi \times 10^2$.

False. The formula uses the radius, so it is $\pi \times 5^2 = 25\pi$ cm$^2$.

What does it mean to give a circle answer "in terms of $\pi$"?

Leave $\pi$ as a symbol in the exact answer instead of using a decimal value.
e.g. area of radius $5$ circle $= 25\pi$ cm$^2$, not $78.5$ cm$^2$.

What are the steps to find the radius of a circle from its area?
e.g. area $= 36\pi$ cm$^2$

1. Write $\pi r^2 =$ area: $\pi r^2 = 36\pi$
2. Divide by $\pi$: $r^2 = 36$
3. Square root: $r = 6$ cm

What are the steps to find the shaded area of a ring between two circles with the same centre?
e.g. radii $3$ cm and $4$ cm

1. Area of large circle: $\pi \times 4^2 = 16\pi$
2. Area of small circle: $\pi \times 3^2 = 9\pi$
3. Subtract: $16\pi - 9\pi = 7\pi$ cm$^2$