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Nested sin and cos equality (Posted on 2023-06-05) Difficulty: 3 of 5
Consider this equation:

sin(sin(sin(sin(x))) = cos(cos(cos(cos(x)))

where x is a real number considered in radians.

Does there exist at least one solution to the given equation?

  • If so, provide all possible solutions to the given equation.
  • If not, prove, with valid reasoning that there does not exist any solution to the given equation.

See The Solution Submitted by K Sengupta    
Rating: 4.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution computer solution | Comment 1 of 2
sym x
fplot(@(x)sin(sin(sin(sin(x)))))
hold on
fplot(@(x)cos(cos(cos(cos(x)))))
grid on

plots the two functions over one cycle.


See the graph here, where are shown somewhat more than the 2*pi cycle length and no intersection is found.

That completes the puzzle which asked only for real solutions.


Wolfram Alpha supplies complex solutions:

x ~= -4.712388980384689857687898276... - 1.210684044891034049791826355... i  
x ~= -4.712388980384689857687898276... + 1.210684044891034049791826355... i
x ~= 0.7568871373475711187264023347... - 0.6101549969203425542976000508... i
x ~= 0.7568871373475711187264023347... + 0.6101549969203425542976000508... i



4.712388980384689857687898276 is 3*pi/2 but Wolfram Alpha wasn't able to identify the other numbers in these solutions.

Edited on June 5, 2023, 2:19 pm
  Posted by Charlie on 2023-06-05 13:43:41

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