# NCERT Class 9 Mathematics Cheat Sheet Triangles Geometry Board Sample Problems

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**Congruence**

Two Geometric figure are said to be congruence if they are exactly same size and shape

Symbol used is

Two angles are congruent if they are equal

Two circle are congruent if they have equal radii

Two squares are congruent if the sides are equal

**Triangle Congruence**

Two triangles are congruent if three sides and three angles of one triangle is congruent to the corresponding sides and angles of the other

Corresponding sides are equal

Corresponding angles are equal

We write this as

The above six equalities are between the corresponding parts of the two congruent triangles. In short form this is called C.P.C.T

We should keep the letters ¡n correct order on both sides

**Different Criterion for Congruence of the triangles**

N | Criterion | Description | Figures and expression |

1 | Side angle Side (SAS) congruence | Two triangles are congruent if the two sides and included angles of one triangle is equal to the two sides and included angle It is an axiom as it cannot be proved so it is an accepted truth ASS and SSA type two triangles may not be congruent always | If following condition Then |

2 | Angle side angle (ASA) congruence | Two triangles are congruent if the two angles and included side of one triangle is equal to the corresponding angles and side It ¡s a theorem and can be proved | If following condition Then |

3 | Angle- angle side (AAS) congruence | Two triangles are congruent if the any two pair of angles and any side of one triangle is equal to the corresponding angles and side It is a theorem and can be proved | If following condition Then |

4 | Side-Side-Side (SSS) congruence | Two triangles are congruent if the three sides of one triangle is equal to the three sides of the another | If following condition Then |

5 | Right angle- hypotenuse- side(RHS) congruence | Two right triangles are congruent if the hypotenuse and a side of the one triangle are equal to corresponding hypotenuse and side of the another | If following condition Then |

Inequalities ¡n Triangles

1) In a triangle angle opposite to longer side is larger

2) In a triangle side opposite to larger angle is larger

3) The sum of any two sides of the triangle is greater than the third side

In triangle ABC

**Some Important points on Triangles**

Orthocenter | Point of intersection of the three altitude of the triangle |

Equilateral | Triangle whose all sides are equal and all angles are equal to |

Median | A line Segment joining the corner of the triangle to the midpoint of the opposite side of the triangle |

Altitude | A line Segment from the corner of the triangle and perpendicular to the opposite side of the triangle |

Isosceles | A triangle whose two sides are equal |

Centroid | Point of intersection of the three median of the triangle is called the centroid of the triangle |

In center | All the angle bisector of the triangle passes through same point |

Circumcenter | The perpendicular bisector of the sides of the triangles passes through same point |

Scalene triangle | Triangle having no equal angles and no equal sides |

Right Triangle | Right triangle has one angle equal to |

Obtuse Triangle | One angle is obtuse angle while other two are acute angles |

Acute Triangle | All the angles are acute |