Question

An AP contains 30 terms. Given that the 10th term is 21 and the sum of the last 10 terms is 675 , find the sum of the first 10 terms .

Answer 1

Answer:

given =

n= 30

a10=21

S30–S20=675

S10= ?

Step-by-step explanation:

we know, a+9d =21

S30-S20=675

30/2(2a+29d) –20/2(2a+19d) =675

15(2a+29d)–10(2a+19d)=675

30a+435d–20a–190d=675

10a+245d=675

multiply 10 to eq 1

10a +90d =210

solving 1 and 2

155d =465

d=3

a=–6

S10=75