Two tangent curves

Calculus Level 3

The graphs of y = ln ( x ) y = \ln(x) and y = x k y = x^k intersect twice if k < Q k < Q and intersects never if k > Q k > Q . What's the value of Q Q ?

1 -1 e e 1 e \frac1e 1 1

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1 solution

James Wilson
Oct 23, 2017

If we let k = 1 / e k=1/e , then the two graphs intersect at ( e e , e ) (e^e,e) , and are also tangent at that point. A function y = x k y=x^k with 0 < k < 1 / e 0<k<1/e will have a value smaller than e e at x = e e x=e^e , be greater than y = ln ( x ) y=\ln(x) at x = 1 x=1 , and will at some point overtake y = ln ( x ) y=\ln(x) , for k > 0 k>0 , because the derivative of y = x k y=x^k is ever-increasing, whereas the derivative of y = ln ( x ) y=\ln(x) is ever-decreasing toward zero. Since both functions are continuous, then, by the intermediate value theorem, they must intersect twice. If k > 1 / e k>1/e , then each function value (in the relevant domain) will be greater than that of k = 1 / e k=1/e , and thus never intersect y = ln ( x ) y=\ln(x) .

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