Sum of $12000 from a financing company for 3 years at 15% per annum. what is the total interest he paid in 3 years

So there are many factors to this, but lets assume the basics:

-He made equal payments every month for 36 months

assuming that is true, the answer is $2,975.42

Assuming that is NOT true and he made NO monthly payments at all and at the end paid the entire balance, the interest would be $6,250.50 

Sin(a)+cos(a)+cot(a)-1=2(csc(x)+1)

Are you trying to figure out the volume?