What is a line of symmetrey?

Answer 1

A line of symmetry (let us call it LoS, for short) defines a boundary where the object in view is the same on both sides of the boundary. The easiest way to visualize an LoS will be draw a random figure on a piece of paper on a table -- an oval perhaps. Take a rectangular mirror and place it within the figure. The portion of the figure that you can see from the glass side of the mirror (not the opaque side) is an exactly replica of the image you see in the mirror. The line where the mirror meets the paper forms an LoS.

In the real world, some objects have LoS's. Most mechanical parts are drawn to be symmetric, for example, so one can make them on a lathe. Electrical circuits can have symmetry too. Same for chemistry -- the molecular structure for H2O, CO2 or CH4, for example. The usefulness of an LOS is that since the portion on one side of the LoS is identical to one on the opposite side, one needs only to treat (calculate area, amount of material to use, measure the angle etc.) one side and assume the same for the other side, reducing the effort for the task.