Câu 7, Ta đặt $(\frac{a}{a-1};\frac{b}{b-1};\frac{c}{c-1})= (x;y;z)$
Do $abc=1\Rightarrow \prod \frac{a}{a-1}= \prod \frac{1}{a-1}= (x-1)(y-1)(z-1)$
hay $xyz= (x-1)(y-1)(z-1)\Leftrightarrow x+y+z=xy+yz+zx+1$
Khi đó $\sum \frac{a^{2}}{(a-1)^{2}}= x^{2}+y^{2}+z^{2}= (x+y+z)^{2}-2(xy+yz+zx)$
$= (x+y+z)^{2}-2(x+y+z-1)= (x+y+z)^{2}-2(x+y+z)+2\geq 1$
Q.E.D