📐 Decode Geometry Notation: Speak the Language of Shapes!

Geometry is the **language of space**! Understanding its terms and notation lets you read diagrams quickly, write proofs clearly, and score marks confidently in O Level exams.

Key Terms & Notation

Point $A$ : a precise position, written with a single capital letter.
Line $\overline{AB}$ : extends infinitely through points $A$ and $B$ .
Line segment $AB$ : the part of a line between $A$ and $B$ (no arrows).
Ray $\overrightarrow{AB}$ : starts at $A$ , passes through $B$ , and continues forever.
Angle $\angle ABC$ : vertex at $B$ , sides $\overline{BA}$ and $\overline{BC}$ .
Parallel lines $AB \parallel CD$ and perpendicular lines $AB \perp CD$ .

Worked Example: Complementary Angles on Intersecting Lines

Problem: At a point, two straight lines intersect forming four angles. One of the angles is labelled

35^{\circ}

. Solution (step-by-step): 1. Opposite (vertical) angles are equal, so another angle is also

35^{\circ}

. 2. The two adjacent angles on a straight line are supplementary:

35^{\circ} + x = 180^{\circ}

3. Solving for

x

gives

x = 145^{\circ}

. 4. The angle complementary to

35^{\circ}

in the right-angle pair is

90^{\circ} - 35^{\circ} = 55^{\circ}

. **Therefore**, the four angles are

35^{\circ}, 145^{\circ}, 35^{\circ}

and

145^{\circ}