Question

in an ap a=5 d=3 an=80 find n and sn

Answer 1

Heya !!!

We know,

$a_{n}$ = a + (n - 1) d

80 = 5 + (n - 1) 3

80 - 5 = (n - 1) 3

75 = (n - 1) 3

75 / 3 = (n - 1)

25 = n - 1

26 = n

and

$S_{n}$ = n / 2 [2a + (n - 1)d]

= 26 / 2 [2(5) + (26 - 1)3]

= 13 (10 + 75)

= 13 × 85

= 1105

Thanks !!