Question

12. Find the sum of n terms of the sequence {an), where an = 5-6n, ne N.

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Answer 1

Step-by-step explanation:

Given: a

n

=5−6n

Find some of the terms of AP:

Put n=1, we get a

1

=−1= first term

Put n=2, we get a

2

=−7= second term

Common difference=d=a

2

−a

1

=−7−(−1)=−6

Sum of first n terms:

S

n

=n/2[2a+(n−1)d]

=n/2[−2+(n−1)(−6)]

=n(2−3n)

Sum of first 20 terms:

S

20

=20/2[2(−1)+(20−1)(−6)]

=10[−2−114]

=−1160

Sum of its first 20 terms of AP is −1160.

Answer 2

Step-by-step explanation:

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