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distinguish between a straight-line distance and a curved-line distance.

A shortest distance covered between two point is called a straight line and  a curve is represented as curve line.

Explanation:

Lines can be short as well as long. A line can be straight as well as curve. A line can be thick as well as thin.  

A straight line is defined as that line which do not have any curves. A straight is one that can be drawn by joining two points or more than two points.

A curve line is that line that is not straight in nature. When there is slight curve in the line it is denoted as the curve line. Example of a curve line is a circle.

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