Giải thích các bước giải:
a.Ta có :
$\dfrac ab=\dfrac cd$
$\to \dfrac ac=\dfrac bd$
$\to\dfrac{a^2}{c^2}=\dfrac{b^2}{d^2}=\dfrac{ab}{cd}=\dfrac{a^2+b^2}{c^2+d^2}$
b.Ta có :
$\dfrac ab=\dfrac cd$
$\to\dfrac{a^2}{b^2}=\dfrac{c^2}{d^2}=\dfrac{ac}{bd}=\dfrac{a^2+c^2}{b^2+d^2}$
c.Ta có : $\dfrac ab=\dfrac cd=k\to a=bk, c=dk$
$\to\dfrac{a^2+3ab}{11a^2-8b^2}=\dfrac{(bk)^2+3bk.b}{11(bk)^2-8b^2}=\dfrac{k^2+3k}{11k^2-8}=\dfrac{(dk)^2+3d^2k}{11(dk)^2-8d^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}$
