<https://byjus.com/ncert-solutions-class-11-maths-chapter-2-relations-and-functions-ex-2-1/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://byjus.com/ncert-solutions-class-11-maths-chapter-2-relations-and-functions-ex-2-1/>