1.
(a6−1)(b6−1)(c6−1)=(a6−a)(b6−b)(c6−c)=abc(6−a)(6−b)(6−c)=abc(b+c)(c+a)(a+b)≥abc8abc=8(dpcm)
2.
A=xy+yz+zx3+x2+y2+z22=2xy+2yz+2zx6+x2+y2+z22=2xy+2yz+2zx2+x2+y2+z22+2xy+2yz+2zx4≥2.(x+y+z)24+xy+yz+zx2≥8+3(x+y+z)22=8+6=14
Do ko xảy ra dấu = nên nó >14