Straight Line — Definition, Formula & Examples
A straight line is a perfectly flat path that extends forever in both directions without any curves or bends. It has no thickness and no endpoints.
A straight line is a one-dimensional figure defined by two distinct points, extending infinitely in both directions with zero curvature. Every point on the line lies along the shortest path between any two of its points.
How It Works
You can draw a straight line by placing a ruler on paper and drawing along its edge. A straight line needs only two points to define it — once you pick any two points, exactly one straight line passes through both. When you name a line, you often use two points on it, like line AB. The angle formed by a straight line is 180°, which is why it is also called a straight angle.
Worked Example
Problem: Points A and B are on a straight line. A ray from point A splits the straight angle into two angles: one measuring 130° and the other unknown. Find the unknown angle.
Step 1: A straight line forms a straight angle, which measures 180°.
Step 2: Subtract the known angle from 180° to find the unknown angle.
Answer: The unknown angle is 50°.
Why It Matters
Straight lines are the building blocks of geometry. Every polygon is made of straight line segments, and understanding lines helps you measure angles, build shapes, and read graphs in math and science classes.
Common Mistakes
Mistake: Confusing a line with a line segment or a ray.
Correction: A straight line extends infinitely in both directions. A line segment has two endpoints, and a ray has one endpoint and extends infinitely in one direction.