Question

If the 10th term of an A.P. is 21 and the sum of its first ten terms is 120, find its nth term.

Answer 1

Answer:

241-22n

Step-by-step explanation:

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Answer 2

$a = 3$

Step-by-step explanation:

given,   n = 10  and  Sn = 120  

            10th term = 21

             a + 9d = 21

             a = 21 - 9d       (equation 1)

  $Sn=\frac{n}{2} {2a+(n-1)d}\\120=\frac{10}{2} {2a+(10-1)d}\\240=10{2a=(9d)d}\\24=2a+9d\\24=2(21-9d)+9d\\24=42-18d+9d\\24-42=-9d\\-18= -9d\\d=2$

put d=2 in equation (i)

$a=21-9d$

$a=21-9\times 2$

$a=21-18$

$a=3$