Question

What is the fifteenth term of the sequence 5,-10,20,-40,80?

Answer 1

here ,

a=5 ; d=-10-5 =-15

then ,

nth term = a+(n-1)d

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

=5-15n+15

=20-15n

now, a15 =20-15×15

=20-225

=-205.