Given : ΔABC in which ∠B = 90 0 and D is the mid point of AC.Ĭonstruction : Produce BD to E so that BD = DE. 5) ΔAOC ≅ ΔBOD 5) SAS postulate 6) AC = BD 6) CPCTC 7) ∠CAO = ∠DBO 7) CPCTC 8) AC || BD 8) If alternate interior angles are congruentĢ) If D is the mid point of the hypotenuse AC of a right triangle ABC, prove that BD = ½ AC. 3) ∠AOC = ∠BOD 3) Vertically opposite angles 4) CO = OD 4) By definition of mid point. 1) Given 2) AO = OB 2) By definition of mid point. Statements Reasons 1) O is the mid point. Prove that : i) ΔAOC ≅ ΔBOD ii) AC = BD and iii) AC || BD. Statements Reasons 1) AB = AC 1) Given 2) AD is a bisector 2) By construction 3) ∠BAD = ∠CAD 3) By definition of angle bisector 4) AD = AD 4) Reflexive (common side) 5) ΔABD ≅ ΔACD 5) SAS Postulate 6) ∠B = ∠C 6) CPCTCġ) O is the mid point of AB and CD. Theorem : Angles opposite to two equal sides of a triangle are equal.Ĭonstruction : Draw the bisector AD of ∠A which meets BC in D. The "SAS" is a mnemonic: each one of the two S's refers to a "side" the A refers to an "angle" between the two sides.Angle in standard position Side Angle Side Postulate Side angle side postulate ->If two sides and the included angle of a triangle are congruent to two sides and the included angle of another triangle, then the two triangles are congruent by side angle side postulate. This is known as the SAS similarity criterion. Any two pairs of sides are proportional, and the angles included between these sides are congruent: ĪB / A′B′ = BC / B′C′ and ∠ ABC is equal in measure to ∠ A′B′C′.This is equivalent to saying that one triangle (or its mirror image) is an enlargement of the other. All the corresponding sides are proportional: ĪB / A′B′ = BC / B′C′ = AC / A′C′.If ∠ BAC is equal in measure to ∠ B′A′C′, and ∠ ABC is equal in measure to ∠ A′B′C′, then this implies that ∠ ACB is equal in measure to ∠ A′C′B′ and the triangles are similar. Any two pairs of angles are congruent, which in Euclidean geometry implies that all three angles are congruent:.There are several criteria each of which is necessary and sufficient for two triangles to be similar: Due to this theorem, several authors simplify the definition of similar triangles to only require that the corresponding three angles are congruent. Note that the "AAA" is a mnemonic: each one of the three A's refers to an "angle". This is known as the AAA similarity theorem. It can be shown that two triangles having congruent angles ( equiangular triangles) are similar, that is, the corresponding sides can be proved to be proportional. Two triangles, △ ABC and △ A′B′C′ are similar if and only if corresponding angles have the same measure: this implies that they are similar if and only if the lengths of corresponding sides are proportional. However, some school textbooks specifically exclude congruent triangles from their definition of similar triangles by insisting that the sizes must be different if the triangles are to qualify as similar. Two congruent shapes are similar, with a scale factor of 1. Corresponding sides of similar polygons are in proportion, and corresponding angles of similar polygons have the same measure. If two angles of a triangle have measures equal to the measures of two angles of another triangle, then the triangles are similar. Figures shown in the same color are similar
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