Suppose f(z) = u(x, y) + iv(x, y) is analytic in some domain
inthe complex plane. show that the families of level curves u(x, y)
=c1 and v(x, y) = c2 are orthogonal. more precisely, at any point
z0of intersection of two leve curves u(x, y) = c1 and v(x, y) = c2
thetangent lines to the two curves are perpendicular, provided f
'(z0)not equal to 0.