Find the extrema and points of inflection for the graph of y=x/(lnx) :
Extrema can only occur at critical points, or where the first derivative is zero or fails to exist.
Note that the domain for the function is x>0, x ne 1 .
y'=(lnx-1)/(lnx)^2 This function is defined for all x in the domain so we set it equal to zero. Note that a fraction is zero if the numerator, but not the denominator, is equal to zero.
lnx-1=0 ==> lnx=1 ==> x=e . For 1
Any inflection point can only occur if the second derivative is equal to zero.
y''=((ln^2x)/x+(2lnx)/x)/(ln^4x) or
y''=(2+lnx)/(xln^3x) Which is negative for x<1, and positive for x>1. The graph is concave down on 0
The graph:
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