đặt a/b =c/d =k⇒a=bk;c=dk
suy ra:ac/bd =bk.dk/bd =k.k=k2
a2+c2/b2+d2 =(bk)2+(dk)/2b2+d2 =b2k2+d2k2b2+d2 =k2.(b2+d2)b2+d2 =k2
vậy ab/bd =a2+c2b2+d2
đặt a/b =c/d =k⇒a=bk;c=dk
suy ra:acbd =bk.dkbd =k.k=k2
a2+c2b2+d2 =(bk)2+(dk)2b2+d2 =b2k2+d2k2b2+d2 =k2.(b2+d2)b2+d2 =k2
vậy abbd =a2+c2b2+d2