利用a²+b²≥（a+b)²/2(x-2)^2+(x-y+1)^2+(3x-y-5)^2=（x-2)^2+(x-y+1)^2+(y-3x+5)^2≥（x-2)^2+（(x-y+1)+(y-3x+5)）²/2…………（ x-y+1=y-3x+5时取得等号）=（x-2)^2+(-2x+6)²/2=3x²...