Question

Find the 20th term of the AP whose sum of the n terms is given by 2n2 + 3n. 1marks​

Answer 1

EXPLANATION.

Sum of Nth term is = 2n² + 3n.

To find 20th term of an Ap.

→ Tn = Sn - S(n - 1)

→ put n = n - 1.

→ 2n² + 3n - [ 2( n - 1)² + 3 ( n - 1) ].

→ 2n² + 3n - [ 2 ( n² + 1 - 2n ) + 3n - 3 ].

→ 2n² + 3n - [ 2n² + 2 - 4n + 3n - 3 ].

→ 2n² + 3n - [ 2n² - n - 1 ].

→ 2n² + 3n - 2n² + n + 1.

→ 4n + 1 = Algebraic expression.

→ put n = 1 = 4(1) + 1 = 5.

→ put n = 2 = 4(2) + 1 = 9.

→ put n = 3 = 4(3) + 1 = 13.

→ put n = 4 = 4(4) + 1 = 17.

→ Series = 5,9,13,17,.......

→ First term = a = 5.

→ Common Difference = d = b - a

9 - 5 = 4.

→ Nth term = An = a + ( n - 1)d.

→ 20th term = a + 19d.

→ 5 + 19(4) = 5 + 76 = 81.

Answer 2

Answer:

the answer is 81

hope it helps you