定理1（介值定理）设函数f(x)在区间[a,b]连续，且f(a)f(b)<0，则方程f(x)=0在区间[a,b]内至少有一个根。(Theorem 1 (intermediate value theorem) set the function f (x) in the interval [a, b] continuous, and f (a) f (b) <0, then the equation f (x) =0 is at least one root in the interval [a and b].)