Cho a/b = c/d, chứng minh rằng: ab/cd = a^2 + b^2 / c^2 + d^2

 Cho a/b=c/d chứng minh rằng :
a)ab/cd=a^2+b^2/c^2+d^2
b)ab/cd=(a-b)^2/(c-d)^2
c)(a+b/c+d)^2=a^2+b^2/c^2+d^2
d)7a^2+3ab/11a^2-8b^2=7c^2+3cd/11c^2-8d^2