Question

if n-th term of an AP is 9-5n then find sum of first fifteen terms of this AP ?​

Answer 1

Answer:

an= 9-5n

a1= 9-5(1)

=4

a2=9-5(2)

=-1

d=a2-a1

=-5

a15=?

a15= a+(n-1)d

=4+(15-1)(-5)

=4+14×(-5)

=4-70

=-66