Question

the sum of first n terms of an A.P.is(3x²+6x).find nth terms and sum of 20 ​

Answer 1

Solution

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s

n

=3n

2

+6n

s

n−1

=3(n−1)

2

+6(n−1)

s

n−1

=3(n

2

−2n+1)+6n−6

s

n−1

=3(n

2

−2n+1)+6n−6

s

n−1

=3n

2

−6n+3+6n−6

s

n−1

=3n

2

−3

t

n

=s

n

−s

n−1

t

n

=3n

2

+6n−3n

2

+3

t

n

=6n+3

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