Question

If nth term of an AP is 2n – 5 , then sum of its first seven terms is

Answer 1

Answer:

Apply formula an=a+(n-1)d

Answer 2

nth term = 2n - 5

=> a + (n - 1)d = 2n - 5

=> a + (n - 1)d = -3 + (n-1)2

comparing lhs and rhs we get

a = -3 and d = 2

sum of 7 terms = 7/2{ 2 x -3 + 6 x 2}

= 7/2 ( 6)

= 21