Cho a/b=c/d a. 5a+3b/5a-3b = 5c+ 3d/ 5c-3d b. (a+c)^2/a^2 - c^2= (b+d)^2 / b^2 - d^2 c. ac/bd = a^2 - c^2 / b^2 - d^2