Question

If sum of n terms is an A.P is given by Sn = 3n2 + 2n then find the 10th term.
1. 55
2. 64
3. 59
4. 49

Answer 1

Correct Answer - Option 3 : 59

CONCEPT :

• Relation between Tn and Sn is given by Tn = Sn - Sn-1.

CALCULATION : 

Given that Sn = 3n2 + 2n.

For Sn-1, we will put (n-1) at the place of n.

⇒ Tn = Sn - Sn-1 = (3n2 + 2n) - {3(n - 1)2 + 2(n - 1)}

⇒ Tn = 3n2 + 2n - {3(n2 - 2n + 1) + 2n - 2}

⇒ Tn = 3n2 + 2n - {3n2 - 6n +3 + 2n - 2}

⇒ Tn = 3n2 + 2n - 3n2 + 4n - 1

⇒ Tn = 6n - 1

Therefore, T10 = 6 ×10 - 1 = 59

Therefore, option (3) is the correct answer.