Question

In an A.P., if the 3rd term is 7 and the sum of its first seven terms is 77, find the sum of its first 20 terms.​

Answer 1

Answer:

if help ful then follow

740

Let a be the first term and d be the common difference of the given A.P. It is given that

a

3

=7 and a

7

=3a

3

+2

a+2d=7 and a+6d=3(a+2d)+2

a+2d=7 and a=−1

a=−1,d=4

Therefore,

S

n

=

2

n

[2a+(n−1)d]

S

20

=

2

20

[2×−1+(20−1)×4]

S

20

=

2

20

(−2+76)=740

Answer 2

a+ 2d = 7

a= 7-2d...

sum of 7 terms = n/2(2a + (n-1)d)) =77

= 7/2 (14-4d + 6d ) = 77

d=4

a= -1

sum of 20 terms = 10(-2 + (19)(4))

=740

Hope it helps..!

Mark brainliest..! (◍•ᴗ•◍)❤