5.2 Intersecting lines · Unit 5 Angles and constructions

1 · Teaching slidesuse ← → keys

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2 · Key words

intersecting linesLines that cross each other.

vertically opposite anglesThe angles opposite each other where two lines cross; they are equal.

perpendicularMeeting at a right angle.

parallelAlways the same distance apart, never meeting.

transversalA line that crosses a pair of parallel lines.

corresponding anglesAngles in matching positions on parallel lines (an F shape); they are equal.

alternate anglesAngles on opposite sides of a transversal (a Z shape); they are equal.

3 · Summary of the sectionrevise from this

Where two straight lines cross, four angles are made.
Vertically opposite angles are equal — the pairs opposite each other.
Angles side by side lie on a straight line, so they add to 180°.
Because of that, the four angles are a, 180−a, a, 180−a — and they total 360°.
If one angle is 90° then all four are, and the lines are perpendicular.
Parallel lines never meet. A line crossing them is a transversal.
On parallel lines, corresponding angles (F shape) are equal and alternate angles (Z shape) are equal.
Always state the reason: “vertically opposite angles are equal”, or “angles on a straight line add to 180°”.