Rearrangement inequality and minimal value of \(\displaystyle{\frac{{{{\sin}^{{3}}{x}}}}{{{\cos{{x}}}}}}+{\frac{{{{\cos}^{{3}}{x}}}}{{{\sin{{x}}}}}}\) For

Rolando Wade

Rolando Wade

Answered question

2022-03-30

Rearrangement inequality and minimal value of sin3xcosx+cos3xsinx
For x(0,π2), is the minimum value of sin3xcosx+cos3xsinx=1? So considering (1cosx,1sinx) and (sin3x,cos3x) is it right to use the rearrangement inequality and conclude that sin3xcosx+cos3xsinx is more than or equal to sin3xsinx+cos3xcosx which is equal to 1?

Answer & Explanation

zalutaloj9a0f

zalutaloj9a0f

Beginner2022-03-31Added 17 answers

Yes it is fine, indeed for x(0,π2) we have in both cases
1. sinxcosxsin3xcos3x1sinx1cosx
then
sin3xcosx+cos3xsinxsin3xsinx+cos3xcosx=1
2. cosxsinxcos3xsin3x1cosx1sinx
then
sin3xcosx+cos3xsinxsin3xsinx+cos3xcosx=1
and equality holds for sinx=cosx that is x=π4

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