Question 389355
{{{a/b = c/d}}} ===> {{{a^2/b^2 = c^2/d^2}}} ====> {{{a^2/b^2 = c^2/d^2 = (a^2 + c^2)/(b^2 + d^2)}}}, using the property of proportions that {{{a/b = c/d = (a+c)/(b+d)}}}.