Let p=a+b+c, q=ab+bc+ca and r=abc, we have
2(p2−2q)+3q=5p or 2p2=5p+q.(1)
We need to prove that 4(p2−2q)+2q+7r≤25 or 4p2+7r≤25+6q.
Since q=p2−5p, the inequality is equivalent to:
7r+30p≤8p2+25.
Notice that (xy+yz+zx)2≥3xyz(x+y+z) implies q2≥3pr. We distinguish two cases regarding the value of p.
* If p=0 then a=b=c=0, so the first inequality is true.
* If p>0 then r≤3pq2, so we have to prove that
73pq2+30p≤8p2+257(2p2−5p)2+90p2≤24p3+75pp(p−3)(2p−5)(14p−5)≤0.(2)
On the other hand, 2p2−5p=q≤3p2 implies 25≤p≤3, so inequality (2) is true. Therefore, the inequality (1) is true.