(f+g)⁺ =f⁺+ g⁺
(f + g)⁻= f⁻+g
if α≥0, (αf)⁺= α f⁺(αf⁻=α f⁻
if α≺0,(αf)⁺ = -αf⁻(α) (-f) ⁻=-αf⁺
prove that i