`a_5 = 190, a_10 = 115` Find a formula for `a_n` for the arithmetic sequence.

We have to find the first term `a_1` and the common difference `d,` then the general formula will be `a_n=a_1+(n-1)d.`

The difference between `a_10` and `a_5` is equal to 5 common differences, i.e.

`d=(a_10-a_5)/5=(115-190)/5=-75/5=-15.`

The first term is 4 common differences less than 5-th, so

`a_1=a_5-4d=190+4*15=250.`

So the answer is: `a_n=250-(n-1)*15.`