Find the equation of the tangent plane to z=xyz = xyz=xy at (1,1,1)(1, 1, 1)(1,1,1).
z=x+y+1z = x + y + 1z=x+y+1
z=x+y−1z = x + y - 1z=x+y−1
z=x−y+1z = x - y + 1z=x−y+1
z=1z = 1z=1