1. Introduction: A Slice of a Round Roti
Picture a big round roti fresh off the tawa. Now cut it like you cut a cake — straight from the centre to the edge, twice — and lift out one wedge. That wedge is a piece of the whole roti.
Here is the question this whole lesson answers: how much roti is in that one wedge? Not the whole roti — just the slice.
A circle is round, so the slice has two straight sides and one curved side. It is not a triangle, not a rectangle. So the area formulas you already know do not fit it directly. That is the difficulty. We need a fresh way to measure a curved slice.
This curved slice of a circle has a name. It is called a sector — the region bounded by two radii (the two straight cuts from the centre) and the arc between them (the curved edge). The word "radii" is just the plural of radius — the straight line from the centre of the circle to its edge.
Stop scrolling. Look at a clock face. The region swept between the hour hand and the minute hand is a sector. Picture it before reading on.
In this lesson you will learn to find the area of any sector, and the length of its curved edge, starting from one simple idea you already trust: a fraction of a whole.
You can now say what a sector is: a slice of a circle, bounded by two radii and the arc between them.