Question

the 5th and 15th terms of an ap is 13 and -17 respectively find the sum of 1st 21 terms of the ap

Answer 1

the answer is above in the photo
am not sure... once u check the answer..
hope this is helpful!!!!

Answer 2

5th = a +(5-1)d = a + 4d = 13 ---------1

15th = a+(15-1)d= a+14d =-17--------2

a + 4d = 13 ---------1
a+14d =-17--------2
------------------------------divided
-10d = 30
d = -3

And a = 13 + 12 = 25

Sum = 21/2 [2×25 + (21-1) -3]
= 21/2 × 15
= 10.5 ×15 = 157. 5