Question

find the value of n if the sum of the first terms of the A.P is 5,23,41........is 70​

Answer 1

Step-by-step explanation:

a=5

d=18

s=70

sn=n/2(2a+(n-1) d)

70=n/2(10+(n-1)18)

70=n/2(10+18n-18)

70=n/2(18n-8)

70=n(9n-4)

70=9n^2-4n

9n^2-4n-70=0

now find n one will be negative so reject it another one will be zero