The illusionist

Algebra Level 3

Find the number of real solutions of x 3 = e x x^3 = e^x .


The answer is 2.

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4 solutions

Otto Bretscher
May 3, 2016

Take logarithms and write 3 ln ( x ) = x 3\ln(x)=x . Now LHS>RHS when x = 3 x=3 but RHS>LHS when x = 1 x=1 and x = e 2 x=e^2 . Thus there are 2 \boxed{2} solutions since the graph of ln ( x ) \ln(x) is concave.

Shivam Mishra
May 11, 2016

is it possible to generalize it for e x = x n e^x=x^n ?

IT SEEMS ANSWER IS 1, BUT NO. IF U SEE THE EQUATION AS e^x/x^3 = 1 and draw its graph by differentiating and checking the nature of graph , the answer is 2

Exactly! :)

Prakhar Bindal - 5 years, 1 month ago

Can we find the real value of x in this case?

Puneet Pinku - 5 years, 1 month ago
Akash Shukla
May 8, 2016

graph of x 3 x^3 is in I and III quadant. While graoh of e x e^x is in I and II quadrant . So the common solution will be in I quadrant only. Now graph of e x e^x starts from ( 0 , 1 ) (0,1) and graph of x 3 x^3 starts from ( 0 , 0 ) (0,0) in I quadrant . Initially for small region slope of e x e^x is less than x 3 x^3 as slope of x 3 x^3 increases very rapidly in square form. So they will intersect in that small region. Now as the x-coordinate increases slope of x 3 x^3 will become less than e x e^x . So again they will intersect. Hence there are two soln.

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