函数f(x)=0,
解 f(x)=0的方程根x
e.g.,
f(x)=ax+b=0 ==>x=-b/x
f(x)=ax*x+bx+c=0 ==>
x=[-b±(b^2-4ac)^(1/2)]/2a
f(x)=asin(x)+b=0 ==> x=arcsin(-b/a)