Question

if the sum of the first n terms of an A.P. is 1/2 [3n square +7n], then find its n'th term. Also, write its 20th term.
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Answer 1

Answer:

Its nth term will be 3n+2

Step-by-step explanation:

Sn=½(3n² +7n)

Sn-1=½{3(n-1)²+7(n-1)}

=½{3(n²-1-2n)²+7n-1}

=½(3n²-3-6n+7n-1)

=½(3n²+n-4)

Tn=(Sn-Sn-1)

=½(3n²+7n)-½(3n²+n-4)

=½(6n+4)

=(3n+2)

Its nth term = (3n+2)

Its 20th term=(3n+2)

=(3×20+2)

=62

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