At a certain point in a material the resultant stress on a plane is \(80 \mathrm{MPa}\) tensile inclined at \(30^{\circ}\) to the normal to the plane. The stress on a plane at right angles to this plane has a normal tensile stress \(50 \mathrm{MPa}\). Find

(a) the resultant stress on second plane,

(b) principal stresses, and

(c) principal planes.