若腊野f(x)在(a,正无穷)上耐简可导lim(x->a+)f(x) = lim(x->正无穷)f(x)=> there exists δ属昌局裤于(a,正无穷), such that f(δ)= min or max of f(x)=>there exists δ属于(a,正无穷) such thatf'(δ) =0
