Question

The sum of the first 21 terms of a linear sequence is 28 and the sum of the first 28 terms is 21 . find which term of the sequence is 0 and also the sum of the terms preceeding it

Answer 1

Use your definitions
"The sum of the first 21 terms of a linear sequence is 28"
---> (21/2)(2a + 20d) = 28
2a + 20d = 8/3

"the sum of the first 28 terms is 21"
---> (28/2)(2a + 27d) = 21
2a + 27d = 3/2

subtract them
7d = -7/6
d = -1/6
2a + 20(-1/6) = 8/3
2a = 6
a = 3

term(n) = 3 + (n-1)(-1/6)
= 3 - n/6 + 1/6
= (18 - n + 1)/6 = (19 - n)/6

so you want (19-n)/6 = 0
19 = n

so the 19th term is 0

now find sum(18)

Answer 2

It’s complicated and complex

Answer 3

I really don't know the answer, that's why I even opened this website