Question

Find the sum of first n terms of an AP whose nth term is (5-6n) Hence find the sum of its first 20 terms ​

Answer 1

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Step-by-step explanation:

answer is in attachment

Answer 2

Answer:

Let an be the nth term of the AP.

∴ an = 5-6n

Putting n = 1,we get

First term, a = a1 = 5-6 × 1 = -1

Putting n = 2,we get

a2 = 5-6 × 2 = -7

Let d be the common difference of the AP.

∴ d = a2 - a1 = -7 - (-1) = -7 + 1 =-6

Sum of first n tern of the AP, Sn

=n2[2×(−1)+(n−1)×(−6)]{sn=n2[2a+(n−1)d]}

=n2[2×(-1)+(n-1)×(-6)] {sn=n2[2a+(n-1)d]}

=n2(−2−6n+6)

=n2(-2-6n+6)

=n(2 -3n)

= 2n -3n2

Putting n = 20,we get

s20

=2×20−3×20×2

=40−1200

=−1160

s20=2×20-3×202=40-1200=-1160

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