How do we know which are the adjacent sides of a triangle ABC when we have to find its area? How do we know which two sides do we have to take as vector a and vector b -- vector AB and vector AC or vector AB and vector BC?

Dear Student, There is no fixed method to determine this.

This totally depends upon the information given in the question and may vary from question to question. In order to explain this please send us a question in which you faced difficulty in determining the sides. We will then explain it you. This will make the learning fruitful and easier. Looking forward to your response.

Cheers!!