Maths puzzle

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Across
  1. 5. A 3 sided polygon.
  2. 6. The line segment connecting two non-adjacent side of a polygon.
  3. 7. The horizontal axis in cartesian coordinate plane.
  4. 8. The union of two rays with a common end-point,called the vertex.
Down
  1. 1. A line that intersects the circle at any one point.
  2. 2. The other name for x-coordinate.
  3. 3. A selection in which order is not important .
  4. 4. Mathematician famous for "Elements of geometry"