Q61 of 84 Page 5

In an A.P., if the 6th and 13th terms are 35 and 70 respectively, find the sum of its 20 terms. (CBSE 2011)

Let a be the first term and d be the common difference,

a6 = a + 5d

35 = a + 5d …. (1)

a13 = a + 12d

70 = a + 12d…. (2)

Subtract (1) from (2),

7d = 35

d = 5

Substitute this value in (1) to get,

35 = a + 5(5)

35 = a + 25

a = 10

S20 = 20/2[2(10)+(20-1)5]

     = 10[20+19(5)]

= 10[20+95]

= 10(115)

=1150

More from this chapter

All 84 →
59

The first and the last terms of an A.P. are 8 and 350 respectively. If its common difference in 9, how many terms are there and what is their sum? (CBSE 2011)

 60

How many multiples of 4 lie between 10 and 250? Also find their sum. (CBSE 2011)

 62

Find the common difference of an AP whose first term is 5 and the sum of its first four terms is half the sum of the next four terms? (CBSE 2012)

 63

Sum of the first 14 terms of an AP is 1505 and its first term is 10. Find its 25th term. (CBSE 2012)