A couple M will be applied to a beam of rectangular cross section from a log of circular cross section. determine the ratio d/b so that maximum stress will be minimum σmax = M(d/2)/((1/12)bd3) σmax = 6M/(b*d2) and d2 + b2 = D2 D =diameter of the circle but i can't find the ratio.. i know the answer of d/b is sqrt 2 how can i get the answer? do i need differentiate? if yes how should i do the differentiation??