Circles, Special Triangles & Quadrilaterals

How the lesson starts: Circles, Special Triangles & Quadrilaterals

Benny's cookie shop
Welcome to Baker Benny's Cookie Shop, where cookies come in every shape! Round cookies need icing piped all around the edge. How much icing? And what do you call a cookie with four equal sides and four right angles? Let's find out!
Icing around a circle
A round cookie has a radius of 5 cm, from the center to the edge. The distance all the way around is the circumference. Why 2 × π × r? Twice the radius is the width across, and the edge is about 3.14 widths long. So 2 × 3.14 × 5 = 31.4 cm of icing!
Let's ice a big cookie
A giant cookie has a radius of 7 cm. Use C = 2 × π × r with 3.14 for π. First, 2 × 3.14 = 6.28. Then 6.28 × 7 = 43.96. Benny needs 43.96 cm of icing. Watch out: forget the 2, and you'd only get 21.98, half the icing!