if n=1, sn=a, if n=2 sn= a+y(a-x), if n=3, sn=a+y2(ysquare)(a-2x) what is formula for sn?

Question

how to calculate the summation? what is the formula? N is not infinity. this is not an infinite series. urgent.

Answer 1

Sn=a+y^(n-1){(a-n+1)x}

Answer 2

either sn=a when n=1 and
for all others the following
formula works. Usually the
first number is given. So the
following is the formula.
sn=a+y^n-1(a-(n-1)x)

Answer 3

It seems
sn = a+y(power n-1)(a-(n-1)x)
General analysis of the given series gives hint about this.