Trigonometry refers to measuring the sides of a triangle. The word comes from the Greek words 'trigon' meaning triangle and 'metron' meaning measurement. Radian is a unit used to measure angles, where one radian is equal to the central angle that cuts off an arc equal in length to the radius of a circle.

1. The word ‘trigonometry’ is derived from the
Greek words ‘trigon’ and ‘metron’ and it means
‘measuring the sides of a triangle’.

5. Relation between radian and real numbers