σ, = 11,55kN/cm²,σ₂ = -11,55kN/cm², a₁ = -15°, máx = 0,00144 У III II 2 5.30°

With respect to planes I, II and III, indicated in the figure, the normal stresses act:
o1 = 10kN/cm^2
o2 = 0
o3 = -10kN/cm^2
Assuming a plane stress state (oz = txy = tyz = 0), determine the principal normal stresses, principal directions and maximum distortion. Additional Data:

E = 20000 kN / cm^2
v = 0.25

Answer (answers in the image) - 
o1= 11.55kN / cm^2
o2= -11.55kN / cm^2
a1 = -15°
ymax = 0.00144