Question

If seventh term of AP is 40. Find the sum of first 13 terms?

Answer 1

Answer:

520

Step-by-step explanation:

Seventh term = 40

=> a(7) = 40

=> a + ( 7 - 1 )d = 40

=> a + 6d = 40

Sum of 13 terms = S(13) = 13/2 × { 2a + ( 13 - 1 )d }

=> S(13) = 13/2 × ( 2a + 12d )

=> S(13) = 13/2 × { 2( a + 6d) }

=> S(13) = 13 × ( a + 6d)

=> S(13) = 13 × 40

=> S(13) = 520

Therefore sum of first 13 terms is 520.