Question

For one arithmetic series T10-T5 = 10 and T10+T5 = 30 then find S100 for that series.​

Answer 1

Answer:

2 T10 = 40

2× a + 9d = 40

a + 9d = 20 -----(1)

- 2 T5 = -20

T5 = 10

a + 4d = 10 ------(2)

On solving ,

5d = 10

d = 2

and

a = 2

after this you can apply sum formula for 100th term,

you have both value d and a that is needed for S100