Geometry Theorems

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Questions and Answers

If angle PQR and angle XYZ are supplementary, and angle PQR is congruent to angle ABC, what can be concluded about angle XYZ and angle ABC?

  • Angle XYZ is congruent to angle ABC. (correct)
  • Angle XYZ is complementary to angle ABC.
  • Angle XYZ is supplementary to angle ABC.
  • Angle XYZ is smaller than angle ABC.

If two lines are intersected by a transversal, and the same-side exterior angles are congruent, then the lines are parallel.

False (B)

State the part-whole postulate for line segments.

The sum of the parts is equal to the whole.

If lines a and b are both parallel to line c, then lines a and b are ________ to each other according to the transitivity of parallel lines.

<p>parallel</p> Signup and view all the answers

Match the angle relationships with their results when a transversal intersects two parallel lines:

<p>Alternate Interior Angles = Congruent Same Side Interior Angles = Supplementary Corresponding Angles = Congruent Vertical Angles = Congruent</p> Signup and view all the answers

In triangle ABC, angle A measures 60 degrees and angle B measures 80 degrees. What is the measure of the exterior angle at vertex C?

<p>140 degrees (B)</p> Signup and view all the answers

Given two triangles, if all three angles of one triangle are congruent to the corresponding angles of the other triangle, then the triangles are congruent.

<p>False (B)</p> Signup and view all the answers

Which criterion can be used to prove that two triangles are congruent if all three sides of one triangle are congruent to the corresponding sides of the other triangle?

<p>SSS (Side-Side-Side) (C)</p> Signup and view all the answers

Which of the following statements is not a direct consequence of the Base Angles Theorem and its converse?

<p>If two triangles share a common side, and two pairs of corresponding angles are congruent, the triangles are congruent. (B)</p> Signup and view all the answers

In any triangle, if an angle bisector is also an altitude, then the triangle must be equilateral.

<p>False (B)</p> Signup and view all the answers

What is the measure of each angle in an equilateral triangle?

<p>60 degrees</p> Signup and view all the answers

The intersection of the angle bisectors of a triangle is known as the ________.

<p>incenter</p> Signup and view all the answers

Match the point of concurrency in a triangle with its definition:

<p>Incenter = Intersection of angle bisectors Orthocenter = Intersection of altitudes Centroid = Intersection of medians</p> Signup and view all the answers

Which of the following conditions cannot guarantee that a triangle is isosceles?

<p>The triangle has no congruent sides. (B)</p> Signup and view all the answers

According to Side-Angle-Side (SAS), if two sides and any angle of one triangle are congruent to the corresponding sides and angle of another triangle, then the triangles are congruent.

<p>False (B)</p> Signup and view all the answers

A right triangle is inscribed in a circle such that the hypotenuse coincides with the diameter of the circle. If the diameter of the circle is 10 cm, what is the length of the median to the hypotenuse?

<p>5 cm (C)</p> Signup and view all the answers

Flashcards

Part-Whole Postulate

The sum of the measures of the parts equals the measure of the whole.

Supplementary Angles

Angles that add up to 180 degrees are supplementary

Congruent Supplements Theorem

Angles supplementary to congruent angles are congruent.

Vertical Angles

Angles opposite each other when two lines intersect.

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Alternate Interior Angles

Angles on opposite sides and inside the parallel lines are congruent.

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Same-Side Interior Angles

Angles on the same side and inside the parallel lines are supplementary.

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Transitivity of Parallel Lines

If line 'c' || 'a' and 'a' || 'b', then 'c' || 'b'.

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Triangle Angle Sum

The sum of the three angles inside equals 180°.

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SAS (Side, Angle, Side)

Two triangles are congruent if two sides and the included angle of one triangle are congruent to the corresponding sides and angle of the other.

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ASA (Angle, Side, Angle)

Two triangles are congruent if two angles and the included side of one triangle are congruent to the corresponding angles and side of the other.

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Base Angles Theorem

The angles opposite the equal sides of an isosceles triangle are equal.

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Base Angles Converse

If two angles in a triangle are congruent, then the sides opposite those angles are congruent (making it isosceles).

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Angles in Equilateral Triangle

All angles in an equilateral triangle are congruent and measure 60 degrees.

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Angle Bisector to Base (Isosceles)

In an isosceles triangle, the angle bisector of the vertex angle is also the median and altitude to the base.

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Radius to Point of Tangency

If a line is tangent to a circle, the radius drawn to the point of tangency is perpendicular to the tangent line.

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Tangent Segments

Tangent segments drawn from the same external point to a circle are congruent.

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Study Notes

  • The sum of the parts equals the whole in geometry.

Angles

  • The Parts-Whole Postulate states the sum of the parts equals the whole.
  • If point B is in the interior of angle AOC, then m∠AOB + m∠BOC = m∠AOC.
  • If m∠AOC is a straight angle, then m∠AOB + m∠BOC = 180 degrees.
  • Angles supplementary to congruent angles are congruent.
  • Vertical angles are congruent.

Parallel Lines

  • Alternate interior/exterior angles are congruent if two lines are intersected by a transversal, then the lines are parallel.
  • Same side interior/exterior angles are supplementary if 2 lines are intersected by a transversal, then the lines are parallel.
  • If parallel lines are intersected by a transversal, then alternate interior and alternate exterior angles are congruent.
  • If parallel lines are intersected by a transversal, then same side interior and same side exterior angles are supplementary.
  • Transitivity of Parallel Lines: If two lines are parallel to a third line, then the two lines are parallel to each other in the same plane.
  • If a line intersects one of two parallel lines in the same plane, it intersects the other line as well.
  • In the same plane, if a line is perpendicular to one of two parallel lines, it is perpendicular to the other line.

Angles in a Triangle

  • Angle Sum: The sum of the measures of the angles in a triangle is 180 degrees.
  • Exterior Angle Theorem: An exterior angle of a triangle equals the sum of the two remote interior angles.
  • Exterior Angle Inequality: An exterior angle of a triangle is greater than either of its remote interior angles.

Congruent Triangles

  • SSS (Side, Side, Side): Two triangles are congruent if all three sides of one are congruent to the corresponding sides of the other.
  • SAS (Side, Angle, Side): Two triangles are congruent if two sides and the included angle of one are congruent to the corresponding sides and angle of the other.
  • ASA (Angle, Side, Angle): Two triangles are congruent if two angles and the included side of one are congruent to the corresponding angles and side of the other.

Isosceles Triangle

  • Base Angles Theorem: The base angles of an isosceles triangle are congruent.
  • Base Angles Converse: If two angles in a triangle are congruent, then the sides opposite those angles are congruent, making the triangle isosceles.
  • Angles in an Equilateral Triangle: The angles of an equilateral triangle are congruent, each measuring 60 degrees.

Angle Bisectors, Altitudes, and Medians in Isosceles Triangles

  • Angle Bisector to Base: In an isosceles triangle, the angle bisector of the vertex angle is also the median and altitude to the base.
  • Median to Base: The median to the base of an isosceles triangle is also an altitude and angle bisector of the vertex angle.
  • Altitude to Base: The altitude to the base of an isosceles triangle is also the median and angle bisector of the vertex angle.
  • Median is also the Altitude: If a median of a triangle is also an altitude, then the triangle is isosceles.
  • Angle bisector is also the Altitude: If an angle bisector of a triangle is also an altitude, then the triangle is isosceles.
  • Angle bisector is also the Median: If an angle bisector of a triangle is also its median, then the triangle is isosceles.
  • Incenter is the intersection of angle bisectors.
  • Orthocenter is the intersection of altitudes.
  • Centroid is the intersection of medians.

Circles, Angles, and Tangents

  • Radius to the Point of Tangency: If a line is tangent to a circle, then the radius to the point of tangency is perpendicular to that line.
  • Tangent Segments: Tangent segments to a circle from the same point are congruent.
  • Triangle inscribed in semi-circle, with one side as a diameter, is a right triangle.
  • Conversely, if a right triangle is inscribed in a circle, it is inscribed in a semicircle, with the hypotenuse as a diameter.

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