An airy stress function is proposed to be: phi = ax^3 + bx^2 y + cxy^2 + dy^3, where sigma_x = partial differential^2 phi /partial differential y^2 = 2cx + 6dy, sigma_y = partial differential^2 phi/partial differential x^2 = 6ax + 2by, tau_xy = -partial differential^2 phi/partial differential x partial differential y = -(2 bx + 2cy) assume a, b, c, d are constants and positive; draw a stress element showing boundary stresses for the following cases: with b=c=d=0 and x-y origin at the left edge of the element and x-y origin at the center of the element with a=d=0 and x-y origin at the center of the element. with the same case as in (2) and c=b=1, determine the boundary forces (that is, normal and shearing forces) on all the edges of a plate measuring 2 m long, 0.5 m wide and 50 mm thick.