Question

10th term of an AP is 13 then sum of 19 terms.

Answer 1

given 10th term of an A.p with a as the first term and d as the common difference



then a10=a+9d=13
now multiply with 2 on both sides....
then we will get 2a+18d=26------(1)

sum of 19 terms in an A.p is n/2[2a+(n-1)d]

so , sum is 19/2[2a+18d] ....
from (1)
19/2(26)
19 × 13
247
therefore the sum is 247

I hope this will help u ;)

Answer 2

Given a10=13
        a10=a+9d=13
now, 
 multiplywith 2 on both sides
we get  2a+18d=26   (1)
sun= n/2(2a+(n-10d)
      =19/2(2a+18d) 
       =19/2(26)
      =19x13
      =247

∴ sum of 19 terms is 247