NCERT Class 9 Maths Formula CBSE Board Sample Problems Part 6
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6. Line and Angles
What is angle: An angle is a formed of two rays with a common endpoint. The Common end point is known as the vertex of the angle and the rays as the sides, sometimes as the legs and sometimes the arms of the angle
Types of Angles
Angle Type | Figure |
Acute Angle | ![]() Acute Angle Loading image••• |
Obtuse Angle | ![]() Obtuse Angle Loading image••• |
Right Angle | ![]() Right Angle Loading image••• |
Reflex Angle | ![]() Reflex Angle Loading image••• |
Straight Angle | ![]() Straight Angle Loading image••• |
S.no | Terms | Descriptions |
1 | Complimentary Angles | Two angles whose sum equal to ![]() Complimentary Angles Loading image••• |
2 | Supplementary Angles | Two angles whose sum equal to 180° ![]() Supplementary Angles Loading image••• |
3 | Vertically Opposite angles | If two lines intersect with each other, then vertically opposite angles are equal ![]() Vertically Opposite Angles Loading image••• |
4 | Transversal across the parallel Lines | If the transversal intersect two parallel lines ![]() Transversal Across the Parallel Lines Loading image••• a) Each pair of corresponding angles are equals b) Each pair of alternate interior angles are equal c) Each pair of interior angles on the same side of the transversal is supplementary |
5 | Theorem on Transversal across the lines | If a transversal intersect two lines such that either a) any one pair of corresponding angles are equal b) any one pair of alternate interior angles are equal c) any one pair of interior angles on the same side of the transversal is supplementary Then the two lines are parallel |
6 | Parallel lines Note | Lines which are parallel to a given line are parallel with each other |
Angles Rules

Angles in a Triangle
Angles in a triangle add up to
So a + b + c =

Angles on a Straight Line
Angles on a straight line add Up to

Angles in a Quadrilateral
Angles in a quadrilateral add up to.
So

Angles Around a Point
Angles around a point add up to.
So
If the side of the triangle is produced, the exterior angle formed is equal to the sum of the opposite interior angle

Triangle is Produced