Question

the sum of arithmetic sequence 15 th term is 495 and 5 th term is 21.
1. what is the 8 th term?
2. common difference?
3. find that sum of first term of this sequence? ​

Answer 1

Answer:

Please recheck the question, as you provided 15th term sum and 5th term sum, which is no way correct!

Answer 2

Answer:

Sn = (n/2)[2a + (n-1)d]

Tn = a + (n – 1)d

S15 = (15/2)(2a + 14d) = 495

Or S15 = 15a + 105d = 495 …. (1)

T5 = a + 4d = 21 …. (2)

From (2), substitute a = 21 – 4d in (1) to get

45d = 180

So d = 4

And a = 21 – 4(4) = 5

So T8 = 5 + 7(4) = 33