Alternate Angles: Definition and Types In this tutorial we will learn definition and types of alternate angles. In geometry, alternate angles are a special kind of angles. Alternate interior angles are formed when a transversal intersect two parallel or non-parallel lines. If a straight line intersects two lines, in the same plane at two distinct […]

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The sum of the angles of a triangle is 180º Given: A triangle △ PQR and ∠1, ∠2, and ∠3 are angles of triangle △ PQR.  To prove:  ∠1 + ∠2 + ∠3 = 1800 Construction: Draw a line XPY parallel to QR passing through P.  Proof: XPY ∥ QR and PQ is transversal ∴ ∠2 = ∠4 (Alternate angles) ….(1) XPY ∥ QR and PR […]

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Theorem – The line segment joining the mid-points of two sides of a triangle is parallel to the third side. Given: A triangle △ABC, in which D and E are mid points of sides AB and AC respectively. DE is joined. To prove: Line joining the mid points D and E (DE) is parallel to the third line […]

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 Theorem : In a parallelogram, opposite angles are equal.  Given: A parallelogram ABCD,  opposite sides of parallelogram are                         side AB and side DC                         side AD and side BC. To prove: Opposite angles of parallelogram are equal. […]

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 Theorem – Alternate Interior Angles are Equal  If a transversal intersects two parallel lines, then each pair of alternate interior angles are equal. Given: Two parallel Lines AB and CD, and PS be transversal intersecting AB at Q and CD at R. To Prove: Each pair of alternate interior angles are equal. i.e. ∠BQR = ∠CRQ and ∠AQR = ∠QRD […]

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If a transversal intersects two lines, such that a pair of alternate interior angles are equal then the two lines are parallel. Given: Two lines AB and CD, and PS be transversal intersecting, AB at Q and CD at R. Each pair of alternate interior angles are equal. i.e. ∠BQR = ∠CRQ and ∠AQR = ∠QRD To Prove: AB ∥ CD Proof: Transversal […]

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If the diagonals of a Quadrilateral, bisect each other then that Quadrilateral is a Parallelogram Given: ABCD is a quadrilateral with AC and BD are diagonals and diagonals intersect each other at O. i.e. OA = OC and OB = OD To Prove: ABCD is a parallelogram. Proof: In △AOD and △COB           […]

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