Question

find the sum of -20 and 21 from the sum of -208 and -40. ​

Answer 1

Answer:

Given: a

2

=8 and a

4

=14 and n=21

We know that,

a

2

=a+d=8...(i)

and a

4

=a+3d=14...(ii)

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

a+d−a−3d=8−14

⇒−2d=−6

⇒d=3

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

a+3=8

⇒a=8−3=5

Now, we have to find the sum of the first 21 terms.

S

n

=

2

n

[2a+(n−1)d]

⇒S

21

=

2

21

[2×5+(21−1)(3)]

⇒S

21

=

2

21

[10+20×(3)]

⇒S

21

=21[5+10×(3)]

⇒S

21

=21[35]

⇒S

21

=735

Step-by-step explanation:

I hope my answer is correct

Answer 2

Answer:

735

Step-by-step explanation:

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