Parallelograms I — O/L Mathematics

What must I remember about Parallelograms I?

• A parallelogram is a quadrilateral with opposite sides parallel.
• Property 1: Opposite sides of a parallelogram are equal in length (AB = CD and AD = BC).
• Property 2: Opposite interior angles of a parallelogram are equal in magnitude (∠A = ∠C and ∠B = ∠D).
• Property 3: Consecutive adjacent interior angles are supplementary, summing to 180° (∠A + ∠B = 180°).
• Property 4: The diagonals of a parallelogram bisect each other into equal halves, dividing the area into two congruent triangles.

Test yourself (1 minute)

1. Formal geometric citations

   When using parallelogram properties in riders, cite 'opp. sides of //gram =', 'opp. angles of //gram =', or 'diagonals of //gram bisect'.

2. Consecutive interior angles

   Remember that consecutive angles add up to 180° because they are allied (co-interior) angles between parallel lines.

Practise Parallelograms I questions today

Must know

• Sufficient conditions to prove a quadrilateral is a parallelogram: Show ONE pair of opposite sides is BOTH equal and parallel, OR both pairs of opposite sides are equal, OR diagonals bisect each other.

Source: NIE G.C.E. (O/L) Mathematics Syllabus (Grade 10, Unit 15)

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Grade 10 · Term 2