Doug's Present and Future Consumption Are Perfect Substitutes

Essay

Doug's present and future consumption are perfect substitutes. He does however discount future consumption by a bit. His utility function is
U(C₀,C₁)= C₀ + C₁/(1 + a)where a is the discount rate he applies to C₁.
i)What is the shape of Doug's indifference curves?
ii)Show that if i exceeds a then C₀ = 0.
iii)Show that if i<a then C₁ = 0.