A triangle is formed by joining three non-collinear points. It has
three sides and three angles. The sums of the angles of a triangle add up
to 180
degree. Triangles are classified according to the relative length
of their sides and measure of angles.
A scalene triangle has all three sides unequal.
An isosceles triangle has two sides equal.
An equilateral triangle has all three sides equal. Each
Interior angle is equal to 600.
An acute triangle is one in which all three interior angles
are acute angles.
An obtuse triangle is one in which one interior angle is an
obtuse angle. Since the sum of all the angles in a triangle is 180, the
other two angles will be acute.
A right-angled triangle is one in which one interior angle is
a right triangle.
Polygons
A polygon is a closed figure in a plane. The number of angles or the
sides identifies the polygons. Thus, a polygon with 3 angles or 3 sides is
a triangle; with 4 angles or 4 sides is a quadrilateral and so on.
Pentagon – 5 sided
Hexagon – 6 sided
Heptagon – 7
sided
Octagon – 8 sided
Nonagon – 9 sided
Decagon – 10
sided
Dodecagon – 12 sided
The different kinds of Polygons are
1. Quadrilaterals
2. Parallelogram
3. Rhombus
4.
Rectangle
5. Square
6. Trapezoid
Examples
1. ABC is a right-angled isosceles triangle, right angled at B, and AB
= BC = a. Line BD is perpendicular to AC. Then BD is equal to:
Solution: Since AB = BC D is the mid
point of AC, i.e., AD = DC
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