Question

if the seventh term of A.P. is 40, then find the sum of the first 13 terms of the A.P.​

Answer 1

Step-by-step explanation:

a7 = 40

a + 6d = 40----------------------( 1 )

S13 = 13/2 [ 2a + (13-1) d]

= 13/2 (2a + 12 d)

= 13

------ × 2 (a+ 6d)

2

=. 13 × 40. (from eq. 1)

=. 520