证明
f(x)=x+1/x
f'(x)=1-1/x^2=(x^2-1)/x^2
令f'(x)>=0
则
(x^2-1)>=0
x∈(负无穷,-1]∪[1,正无穷时f(x)递增
所以
f(x)在【-3,-1】单调递增
所以
f(x)MAX=F(-1)=-2
f(x)MIN=f(-3)=-10/3