A consumer has a utility function u(x₁, x₂)= 3(x₁²+ x₂²) and the budget constraint is

p₁x₁+ p₂x₂=I , p₁ , p₂ , I> 0 . Check the Roy's identity _i(p, I)= -{v(p, I)/ p_i} {v(p, I)/ I }

for i=1, 2. ( p,I) is the Marshallian demand function, v =v( p,I) is the indirect utility function.