What is the purpose of Ethos, Logos, and Pathos?

I'm doing my summer reading with the book called THE CRUCIBLE, one of my ignments is to find quotes that demonstrate Ethos, Logos, and Pathos, i google them and I understand the definition, but i still don't get what's the purpose of them or HOW AM I SUPPOSED TO FIND A QUOTE THAT REPRESENTS THEM IN THE BOOK? help me 10 points