In this case, we have to first find the number of terms.
Here a = 101, l = Tn = 47, d = 99 – 101 = –2
∴ 47 = 101 + (n – 1) × (–2), where n = number of terms
⇒ 47 = 101 – 2n +2
⇒ 2n = 103 – 47 = 56 ⇒ n = 28
∴ Sn = \(\frac{n}{2}\) (a + l)
⇒ S28 = \(\frac{28}{2}\) (101 + 47)
= 14 × 148
= 2072.