Class 9: Rectilinear Figures – Lecture Notes – ICSE / ISC / CBSE Mathematics Portal for K12 Students

This chapter is based on polygons. We had published lecture notes on Polygons for Class 8 students. It would be a good idea to revise the Class 8: Polygons – Lecture Notes.

Important points to remember:

The sum of the interior angles of a convex polygon of $n$ sides is $(2n-4)$ right angles or $(2n-4) \times 90^o$ .
If there is a regular polygon of $n$ sides $(n \ge 3)$ , then each of its interior angle is equal to $\frac{2n-4}{n}$ $\times 90^o$
Each exterior angle of a regular polygon of $n$ sides is equal to $($ $\frac{360}{n}$ $)^o$
If each exterior angle of a regular polygon is $x^o$ , then the number of sides in the polygon is $\frac{360}{x}$
As the number of sides of a regular polygon increase , the measure of each interior angle also increases.
If the polygon has $n$ sides, then the number of diagonals in $\frac{n(n-3)}{2}$ .
The sum of all exterior angles formed by producing the sides of a convex polygon in the same order is equal to four right angles (or $360^o$ ).