SOLUTION: We know that the graph of a logarithmic function is symmetrical to the graph of its corresponding exponential function with respect to the line y=x. Will the two graphs intersect w

Algebra ->  Graphs -> SOLUTION: We know that the graph of a logarithmic function is symmetrical to the graph of its corresponding exponential function with respect to the line y=x. Will the two graphs intersect w      Log On


   



Question 1172409: We know that the graph of a logarithmic function is symmetrical to the graph of its corresponding exponential function with respect to the line y=x. Will the two graphs intersect with each other? If so, how many points of intersection will there be? Will each point of intersection lie on the line y=x? Explain please.
Answer by math_helper(2461) About Me  (Show Source):
You can put this solution on YOUR website!

Looking at base = e, and ln (the natural log)...
A graph showing y+=+e%5Ex, y+=+x, and +y+=+ln%28x%29+ is a quick way to see they will never intersect.

Aside from that... for +x%3C0, clearly e%5Ex+%3E+x+ as +e%5Ex+%3E0+ while x%3C0. For the half-plane x>=0, one can write the Taylor series expansion for e%5Ex:
e%5Ex = sum%28%28x%5En%2Fn%21%29%2C+n=0%2C+infinity%29
Pull out the first two terms from the RHS:
e%5Ex = 1+%2B+x+%2B+sum%28%28x%5En%2Fn%21%29%2C+n=2%2C+infinity%29
So clearly e%5Ex+%3E+x+ for +x%3E=+0 and combined with the first part, e%5Ex+%3E+x+ for all x.

To prove +ln%28x%29+ < x+ (here the domain of x is x>0, as ln(x) is not defined for x%3C=0 ), we can use the result from above as the starting point:
+e%5Ex++%3E++x
Take ln() both sides:
+ln%28e%5Ex%29+ > +ln%28x%29+
++x+ > +ln%28x%29+ , x>0