I want to prove this equality holds: \(\displaystyle{\tan{{\left({\frac{{\pi}}{{{7}}}}\right)}}}{\tan{{\left({\frac{{{2}\pi}}{{{7}}}}\right)}}}{\tan{{\left({\frac{{{3}\pi}}{{{7}}}}\right)}}}=\sqrt{{{7}}}\)

Darian Silva

Darian Silva

Answered question

2022-04-06

I want to prove this equality holds:
tan(π7)tan(2π7)tan(3π7)=7

Answer & Explanation

Emelia Leon

Emelia Leon

Beginner2022-04-07Added 18 answers

Note that
tanπ7tan2π7tan3π7=sinπ7sin2π7sin3π7cosπ7cos2π7cos3π7
Now, it is trivial to see that
3π7=π7+2π7=π7+π7+π7
Thus, concentrating on the numerator and using the identity
sin(α±β)=sinαcosβ±cosαsinβ
We find
sinπ7sin2π7sin3π7=sinπ7(sinπ7cosπ7+cosπ7sinπ7)sin3π7
=2sin2π7cosπ7sin3π7
Now repeat this process for the term on 3π7 and continue into the denominator and you should be ok.

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