Question

Find Sn of the A.P 8,6,4.....​

Answer 1

Answer:

Given A.P is 24,20,16,...

so, the first term, a=24

the common difference, d=20−24

=−4.

The sum of first n terms of A.P is given by

2

n

[2a+(n−1)d]=

2

n

[48−(n−1)4]

Given, the sum is 72.

so,

2

n

[48−(n−1)4]=72

⇒24n−n(n−1)2=72

⇒2n

2

−26n+72=0⇒n

2

−13n+36=0

⇒n=4 or 9.

∴ 4 terms and 9 terms add upto 72.

solution

Answer 2

Answer:

a = 8

d = 6-8 = -2

sn = n/2 [2a+(n-1)d]

= n/2[2×8 + (n-1)-2]

= n/2 [16 -2n + 2]

= n/2 [18-2n]

= n/2 2[7-n]

= n(7-n)

= 7n-n²

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