Question

solve this problem . It is from A.P 10th class

Answer 1

write the question properly .can't understand your handwriting

Answer 2

given = $a_{12} = 81$
A.P = x-1, x-2, x+3.......
answer,
a = x-1
d = x-2 - (x-1)
   = -1
a + 11d = 81
(x-1) + 11(-1) = 81
x = 81+12 =93
$a_{n} = a + (n-1)d$
=(x-1) + n-1(-1)
= 92-n +1
$a_{n} = 93 - n$
now,
$S_{n} = \frac{n}{2} (a + l)$
=$\frac{n}{2} ( 92 + 93 - n)$
$= \frac{185n - n^{2} }{2}$ .... this is the last answer

I hope this will help you!!!!