Question

7th term of an ap is 40,then the sum of first thirteen terms is.

Answer 1

Answer:

S13 = 520

Step-by-step explanation:

tn = a + ( n - 1 ) d

t7 = a + ( 7 - 1 ) d

t7 = a + 6d

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

Sn = n ÷ 2 [ 2a + ( n - 1 ) d ]

S13 = 13 ÷2 [ 2a + 12d ]

S13 = 13 × a + 6d

But , a + 6d = 40

S13 = 13 ×40

S13 = 520