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## Straight Line

Join two points A,B by constructing a straight line that they both lie upon terminating at their respective locations.

Straight line constructed by positioning

Straight line constructed by positioning

**straight edge**touching both locations.## Angles

Consider 2 straight lines AB, CD intersecting at O
AO can be rotated about O so that it lies upon CO, occupying the same set of points The amount of revolution is known as the angle AOCIf AO continues to rotate about O until it once again overlays its original position it will have performed 1 revolution. One measure divides 1 revolution into 360 degrees, each degree into 60 minutes and each minute into 6o seconds i.e. 1 revolution = 360 degrees = 2,160 minutes = 1,296,000 seconds (5º is shorthand way of writing 5 degrees) |

## Right Angles

CD is a straight line.
O is a position that lies on CD and AO is another straight line. ∠AOD is the amount a line OD needs to be rotated to position it over OA. (“∠” is the symbol for angle) All values for this angle are possible, including the one that satisfies ∠AOD = ∠AOC This angle is called a right angle (rt. ang.) and AD is said to be perpendicular to CDIf one rotates AO another 1 rt. ang. anti-clockwise it lies on CO since ∠AOD = ∠AOC = 1 rt. ang This is equivalent to rotating OD through 2 rt. ang.s. But CO is part of straight line CD, so a straight line may be thought of the addition of 2 rt. ang.s. BO is drawn such that ∠COB = ∠BOD then as before these are rt. ang.s then as AB is also built of 2 rt. angs. it is also a straight line. So 4 rt. ang.s = 1 revolution = 360º ∴ 1 rt. ang. = 90º (Currently we have shown how a rt. ang. can exist, later it will be shown how to construct one) |

## Opposite angles of intersecting lines are equalAB & CD are two straight lines intersecting at O. Let MN be a line passing through O perpendicular to AB ∴ ∠MOB = ∠NOB = ∠AON = ∠AOM = 1 rt. ang. ∠DOB = 1 rt. ang. - ∠NOD ∠AOC = 2 rt. ang.s - ∠AOD = 2 rt. ang.s - 1 rt. ang. - ∠NOD = 1 rt. ang. - ∠NOD = ∠DOB Similalry ∠DOA = ∠COB AB & CD are two straight lines intersecting at O. Let MN be a line passing through O perpendicular to AB ∴ ∠MOB = ∠NOB = ∠AON = ∠AOM = 1 rt. ang. ∠DOB = 1 rt. ang. - ∠NOD ∠AOC = 2 rt. ang.s - ∠AOD = 2 rt. ang.s - 1 rt. ang. - ∠NOD = 1 rt. ang. - ∠NOD = ∠DOB Similalry ∠DOA = ∠COB |