Question

If the nth term of na ap -1,4,9,11,... Is 129 find the value,of n

Answer 1

ANSWER

S

n

=

2

n

{2a+(n−1)d}

S

n−1

=

2

n−1

{2a+(n−2)d}

S

n

−S

n−1

=

2

n

{2a+(n−1)d}−

2

n−1

{2a+(n−2)d}

S

n

−S

n−1

=

2

2an

+

2

n(n−1)d

−

2

2an

+

2

2a

−

2

(n−1)

(n−2)

S

n

−S

n−1

=a+(n−1)d=T

n

∴T

n

=(S

n

−S

n−1

)=(3n

2

+5n)−(3n

2

−n−2)=(6n+2).

T

m

=164⇒6m+2=164⇒6m=162⇒m=27.