`a_1 = 4 `

`d = 3`

`n = 10`

`S_n = n/2[2a_1 + (n − 1)d] = 10/2[2 × 4 + 9 × 3] = 175`

Can you see why we didn't use the first formula? We don't know the final term (but actually, this is no big deal - it is easy to find.)