Show that ln(x)≤x−1
I'm not really sure how to show this, it's obvious if we draw a graph of it but that won't suffice here. Could we somehow use the fact that ex is the inverse? I mean, if ex−1≥x then would the statement be proved?
Answer
Define for x>0
f(x)=lnx−x+1⟹f′(x)=1x−1=0⟺x=1
and since f″(x)=−1x2<0∀x>0 , we get a maximal point.
But also
limx→0+f(x)=−∞=limx→∞f(x)
Thus, the above is a global maximal point and
∀x>0,f(x)≤f(1)=0
No comments:
Post a Comment