a 1+a2=5

a2=5-a 1

a2+a3=5

a3 = 5-a2 = 5-(5-a 1)= a 1

a3+a4=5

a4=5-a3=5-a 1=a2

...

a 18=a2=5-a 1=5-2=3

When n is an even number

Sn=a 1+a2+a3+...+ An

=n/2a 1+n/2a2

=n/2(a 1+a2)

=5n/2

When n is an odd number

Sn=a 1+a2+a3+...+ An

=(n+ 1)/2a 1+(n- 1)/2 a2

=(n+ 1)/2a 1+(n+ 1-2)/2(5-a 1)

=(n+ 1)/2a 1+(n+ 1)/2 * 5-(n+ 1)/2a 1-5+a 1

=5/2(n+ 1)-5+a 1

=5/2n-5/2+2

=5/2n- 1/2