Q24 of 202 Page 9

Find the sum of the first 15 terms of each of the following sequences having nth term as

(i)


(ii)


(iii)


(iv)

(i)


Put n = 1


a1 = 3 + 4(1) = 7


Put n = 15


a15 = 3 + 4(15) = 63 = l


Sum of 15 terms, S15 = [a + l]


= [7 + 63] = 525


(ii)


Put n = 1


b1 = 5 + 2(1) = 7


Put n = 15


b15 = 5 + 2(15) = 35 = l


Sum of 15 terms, S15= [a + l]


= [7 + 35] = 315


(iii)


Put n = 1


x1 = 6 – 1 = 5


Put n = 15


x15 = 6 – 15 = -9


Sum of 15 terms, S15= [a + l]


= [5 - 9] = -30


(iv)


Put n = 1


y1= 9 – 5(1) = 4


Put n = 15


y15= 9 – 5(15) = -66


Sum of 15 terms, S15= [a + l]


= [4 - 66] = -465


More from this chapter

All 202 →
22

Find the sum of

(i) The first 15 multiples of 8


(ii) The first 40 positive integers divisible by (a) 3 (b) 5 (c) 6.


(iii) All 3 – digit natural numbers which are divisible by 13.


(iv) All 3 – digit natural numbers, which are multiples of 11.

 23

Find the sum :

(i)


(ii)


(iii)


(iv)


(v)


(vi)


(vii)


(viii)

 25

Find the sum of first 20 terms of these sequence whose nth term is .

 26

Find the sum of first 25 terms of an A.P. whose nth term is given by .