f(x)=1+1/x
设10.
f(x1)-f(x2)
=(1+1/x1)-(1+1/x2)
=1/x1-1/x2
=(x2-x1)/(x1x2)
>0
即f(x1)>f(x2),f(x)在区间[1,+无穷)上单调递减.
.