Question

26. The sum of first 13 terms of an A.P. is 21 and the sum of
first 21 terms is 13. Determine, the sum of first 34 terms​

Answer 1

Answer: thirteenth terms of AP use sum formula.then add it with sum of 21ternsat last use sum fun formula.so simple

Step-by-step explanation:

Answer 2

Step-by-step explanation:

s13=21

13/2(2a+12d)=21

2a+12d=24/13

divide by 13

26a+156d=24

13a+78d=12 --------eq.1

s21=13

21/2(2a+20d)=13

2a+20d=26/21

a+10d=13/21

divide by 21

21a+210d=13 eq......2