Question

Find the sum of the arithmetic progression of 11, 5,-7,-1

Answer 1

Answer:

a=11,d=5-11= -6, Sn=?, n=?,an= -1

an=a+(n-1)d

-1= 11(n-1)-6

-11-1= -6n+6

-12-6= -6n

-18= -6n

n= -18/-6

n=3

Sn=2a+(n-1)d

=22+2×-6

=22-12

=10

Sn=10