Which function represents exponential decay?
f(x) =1/2 (3/2)^x
f(x) =1/2 (-3/2)^x
f(x) =4 (-2/3)^x
f(x) = 4 (2/3)^x
f(x) =1/2 (3/2)^x
f(x) =1/2 (-3/2)^x
f(x) =4 (-2/3)^x
f(x) = 4 (2/3)^x