Question

Given nth terms of the series, find the sum to 2n terms.

(i) 3n2 + 2n + 1 (ii) n3 + 2n + 3​

Answer 1

Answer:

S

1

=2(1)+3(1)

2

=5

S

n

=

2

n

{2a+(n−1)d}

S

n

=

2

n

{a+a+(n−1)d}

S

n

=

2

n

{a+a

n

}

2n+3n

2

=

2

n

{5+a

n

}

4+6n=5+a

n

⇒a

n

=6n−1

⇒a

r

=6r−1