二次函数f(x)=ax^2+bx+c
x=1时的函数值即为f(1)=a+b+c
x=-2时的函数值即为f(-2)=4a-2b+c
f(0)=c
f(1/2)=a/4+b/2+c
f(-1/2)=a/4-b/2+c
f(1/2)-f(0)=a/4+b/2,得：a+2b=4[f(1/2)-f(0)]
f(-1/2)-f(0)=a/4-b/2,得：a-2b=4[f(-1/2)-f(0)]