Cho \(\dfrac{a}{b}=\dfrac{c}{d} \) . Chứng minh :
a, \(\dfrac{a^2+c^2}{b^2+d^2} =\dfrac{ac}{bd}\)
b, \(\dfrac{a^2+c^2}{b^2+d^2} = \dfrac{a^2-c^2}{b^2-d^2}\)
c, \(\dfrac{(a+c)^2}{(b+d)^2} = \dfrac{(a-c)^2}{b-d)^2}\)
d, \(\dfrac{a^2+b^2}{a^2-b^2} = \dfrac{c^2+d^2}{c^2-d^2}\)
e, \(\dfrac{(a-b )^2}{(c-d)^2} = \dfrac{a^2+b^2}{c^2+d^2}\)