First formula
If you have to buy any car , tv , or any other product , then this formula can used for calculating value of annuity . In many questions , you will see that buyer will pay some amount when he buys product , so deduct it from principle amount . If number of installement is more then you can use logarithm( learn here and here ) for solving these type of questions
Second formula
If you have created any fund to buy new fixed asset then second formula is more simple than first
