NCERT Class 9 Maths Formula CBSE Board Sample Problems Part 7
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7. Triangles
S.no | Terms | Descriptions |
1 | 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 |
2 | 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  Triangle Congruence Loading image••• Corresponding sides are equal AB=DE, BC=EF, AC=DF 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 in correct order on both sides |
3 | Inequalities in 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 |
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 |  Side Angle Side (SAS) Congruence Loading image••• If following condition AB=DE, BC=EF Then |
2 | Angle side | Two triangles are congruent if the angle (ASA) two angles and included side of congruence one triangle is equal to the corresponding angles and side It is a theorem and can be proved | If following condition Then |
3 | Angle angle side (AAS) congruent | Two triangles are if the any two pair of angles and any congruence side of one triangle is equal to the corresponding angles and side It is a theorem and can be proved |  Side-Side-Side (SSS) Congruence Loading image••• If following condition BC = EF Then |
4 | Side-Side-Side (SSS) congruence | Two triangles are congruent if the three sides of one triangle is equal A to the three sides of the another |  Side-Side-Side (SSS) Congruence Loading image••• If following condition BC= E F,AB= DE, DF =AC Then |
5 | Right angle hypotenuse-side (RHS) congruence | Two right triangles are congruent if A the hypotenuse and a side of the one triangle are equal to corresponding hypotenuse and side of the another |  Right Angle Hypotenuse-Side (RHS) Congruence Loading image••• If following condition AC= DF, BC = EF Then |
Some Important Points on Triangles
Point of intersection of the three altitude of the triangle | |
Orthocenter | Triangle whose all sides are equal and all angles are equal to 60° |
Equilateral | A line Segment joining the corner of the triangle to the midpoint of the opposite side of the triangle |
Median | A line Segment from the corner of the triangle and perpendicular to the opposite side of the triangle |
Altitude | A triangle whose two sides are equal |
Isosceles 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 |