Question

the mathematical system named after an ancient greek mathematician, which includes principles outlined in the \elements\, is called geometry.
geometry based on axioms different from those of classical euclidean geometry, often involving curved spaces, is called geometry.
visual images that are designed to trick the eye into seeing something different from what is actually there are known as illusions.
the basic colors from which all other colors can be obtained through mixing are called colors.

Explanation:

1. Euclid, an ancient Greek mathematician, wrote Elements which outlines the principles of Euclidean Geometry.
2. Non - Euclidean Geometry is based on axioms different from classical Euclidean Geometry and often involves curved spaces.
3. Optical illusions are visual images that deceive the eye into seeing something different from reality.
4. Primary colors are the basic colors from which all other colors can be obtained through mixing.

Answer:

1. Euclidean
2. Non - Euclidean
3. optical
4. primary