Question

25. In an AP,if a=1, an=19 and
n=10 then find Sn:
O 80
O 90
O 100
O none​

Answer 1

Answer:

we have, a=1 , an = 19 and n= 10 .

an= a+(n-1) d

19 = 1 +(10-1) d

19= 1+ 9d

19 - 1 = 9d

18 = 9d

d = 18/9

d = 2

Now, sn = n/2[2a+(n-1) d]

= 10/2[2x1+(10-1) 2

= 5 (2+18)

= 5x20

= 100

hence, answer is 100 .