First term t(1)=(x-y) common ratio =(x+y) (don't omit the parentheses) then the nth term: t(n)=(x-y)(x+y)^(n-1) check: t(1)=(x-y)(x+y)^(1-1)=(x-y)(x+y)^0=(x-y)(1)=(x-y) t(3)=(x-y)(x+y)^(3-1)=(x-y)(x+y)^2
When n is small, t(n) can also be found by repeated multiplication of the common ratio (x+y)
