Which series has a common ratio r=−0.5r = -0.5r=−0.5?
1,−0.5,0.25,−0.125,…1, -0.5, 0.25, -0.125, \dots1,−0.5,0.25,−0.125,…
1,0.5,0.25,0.125,…1, 0.5, 0.25, 0.125, \dots1,0.5,0.25,0.125,…
1,−2,4,−8,…1, -2, 4, -8, \dots1,−2,4,−8,…
1,0.5,−0.25,0.125,…1, 0.5, -0.25, 0.125, \dots1,0.5,−0.25,0.125,…