ab+bc+ca=abc\Rightarrow 1/a+1/b+1/c=1
ma 1/a +1/b+1/c\geq 9/(a+b+c)\Rightarrow(a+b+c)\leq9
1/a(a-1)=1/(a-1)-1/a
\RightarrowP=(1/(a-1)+1/(b-1)+1/(b-1))-(1/a+1/b+1/c)
=(1/(a-1)+1/(b-1)+1/(b-1)) - 1
ta co: (1/(a-1)+1/(b-1)+1/(b-1))\geq 9/(a+b+c-3)=9/6=3/2 (Svac-xo)
\RightarrowP\geq3/2-1=1/2\Rightarrowdpcm
hehe em moi hoc ve cai nay nen bai lam dai thong cam nha