c) The sum to infinity of a GP is twice the sum of the first two terms. Find possible values of the common ratio

Answer 1

S = a + a r
so 2S = 2 a + 2 a r
and
2 S = a / (1+r)
so
2 a + 2 a r = a /(1+r)
4 = 1/(1+r)
4 + 4 r = 1
r = -3/4