a₁ = 4

d = 3

n = 10

S_n = n/2[2a₁ + (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.