(a+c)/b=1;
b=a+c;
b^2=a^2+2ab+c^2=a^-2ac+c^2+4ac=(a-c)^2+4ac;
(a-c)^2>=0
所以:b^2>=4ac
b=a+c
b^2=a^2+c^+2ac=(a-c)^2+4ac>=4ac