Calculate the following limits or show they do not exist.
a) lim₍ₓ,ᵧ₎→₍₋₂,₁₎→(-2,1)4x²y³/2x³-y⁹
b) lim₍ₓ,ᵧ₎→₍₀,₀₎→(0,0)4x²y³/2x³-y⁹
c) lim₍ₓ,ᵧ₎→₍₀,₁₎→(0,1)xy-x/x²+(y-1)²