ab+bc+ca = 1/2{(a+b+c)^2-(a^2+b^2+c^2)} = 1/4{2(a+b+c)^2-(a^2+b^2)-(b^2+c^2)-(c^2+a^2)}= 1/2(a+b+c)^2 - 1/4{(a^2+b^2)+(b^2+c^2)+c^2-a^2)}= 1/2(a+b+c)^2 - 1/4(1+2+2)= 1/2(a+b+c)^2 - 5/41/2(a+b+c)^2≥01...