The first and last terms of an A.P. are 1 and 11 respectively. If sum

1.	The first and last terms of an A.P. are 1 and 11 respectively. If sum of its terms is 36 then number of terms will be :(A) 5(B) 6(C) 9(D) 11
Answer» Answer is (B) 6 a= 1, l = 11, S_n = 36 S_n = n/2(a + l) 36 = n/2(1 + 11) ⇒ 36 = n/2 × 12 ⇒ 36 = 6n ⇒ n = 6

The first and last terms of an A.P. are 1 and 11 respectively. If sum of its terms is 36 then number of terms will be :(A) 5(B) 6(C) 9(D) 11

Answer»

Answer is (B) 6

a= 1, l = 11, S_n = 36

S_n = n/2(a + l)

36 = n/2(1 + 11)

⇒ 36 = n/2 × 12

⇒ 36 = 6n

⇒ n = 6