Adjacent
Example
Problem: In quadrilateral ABCD, identify which sides are adjacent to side AB.
Step 1: List all four sides of the quadrilateral: AB, BC, CD, and DA.
Step 2: Find the sides that share an endpoint (vertex) with AB. Side AB has endpoints A and B.
Step 3: Side BC shares vertex B with side AB, so BC is adjacent to AB. Side DA shares vertex A with side AB, so DA is adjacent to AB. Side CD shares neither endpoint with AB, so it is not adjacent — it is the opposite side.
Answer: Sides BC and DA are adjacent to side AB.
Why It Matters
The concept of adjacency appears throughout geometry and beyond. Recognizing adjacent sides helps you apply formulas for perimeter, use the law of cosines on triangles, and identify angle relationships. In graph theory and computer science, adjacency describes which nodes are directly connected, forming the basis of network analysis.
Common Mistakes
Mistake: Confusing adjacent with equal or congruent.
Correction: Adjacent only means "next to" — two adjacent sides or angles can have completely different measurements. Being adjacent describes position, not size.
Related Terms
- Adjacent Angles — Angles that share a common side and vertex
- Non-Adjacent — The opposite — not next to each other
- Vertex — The shared point where adjacent sides meet
- Complementary Angles — Often formed by adjacent angle pairs