Question

the sum of first 35 terms of an arithmetic sequence is 700, find the sum of first and 35th term​

Answer 1

Answer:

a

2

=2 and a

7

=22 and n=35

We know that,

a

2

=a+d=2...(i)

and a

7

=a+6d=22...(ii)

Solving the linear equation (i) and (ii), we get

a+d−a−6d=2−22

⇒−5d=−20

⇒d=4

Putting the value of d in equation. (i), we get

a+4=2

⇒a=2−4=−2

Now, we have to find the sum of first 35 terms.

S

n

=

2

n

[2a+(n−1)d]

⇒S

35

=

2

35

[2×(−2)+(35−1)4]

⇒S

35

=

2

35

[−4+34×4]

⇒S

35

=35[−2+34×2]

⇒S

35

=35[66]

⇒S

35

=2310