If a machine part is made of titanium (Ti-6A1-4V) and a critical point in the material is subjected to plane stress, such that the principal stresses are σ₁ and σ₂ = 0.5 σ₁, determine the magnitude of σ₁ in MPa that will cause yielding according to (a) the maximum shear stress theory, and (b) the maximum distortion energy theory.