Question

????????????????????????????????????????????

Answer 1

`a_n=19-4n`

Explanation:

If
`a_1=15`, then the second term is:

`a_2=a_(2-1)-4\\a_2=a_1-4\\a_2=15-4\\a_2=11`

This shows that our common difference is
`d=-4`, so we can write the explicit formula:

`a_n=a_1+(n-1)d\\a_n=15+(n-1)(-4)\\a_n=15-4n+4\\a_n=19-4n`