Question

an=2n2+1 then find ‘d’.​

Answer 1

Given a

n

=2n

2

+1 (1)

Put n=1 in (1), we get,

a

1

=2(1)

2

+1

∴a

1

=2+1

∴a

1

=3

Put n=2 in (1), we get,

a

2

=2(2)

2

+1

a

2

=2(4)+1

∴a

2

=9

Thus, d=a

2

−a

1

∴d=9−3

∴d=6

Now, sum of n terms of A.P. is,

S

n

=

2

n

[2a+(n−1)d]

Put n=2 in above equation, we get,

S

2

=

2

2

[2a+(2−1)d]

∴S

2

=1[(2×3)+(1×6)]

∴S

2

=1[6+6]

∴S

2

=12