Trig or treat

Geometry Level 5

Find the sum of the first 3 lowest positive values of x in degrees.


The answer is 65.

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1 solution

Aritra Jana
Oct 22, 2014

the main problem with this problem is that it looks tremendously horrible at first sight. i cannot find any good method to solve this. if you have, please do comment


First we write 3 x = Z 3x=Z .. (for simplicity, obviously)

then we write s e c 2 Z = 1 + t a n 2 Z sec^{2}Z=1+tan^{2}Z , bring all the terms to the L.H.S, group them, to get our equation:

3 t a n 3 Z + ( 2 3 ) t a n 2 Z ( 2 + 3 ) t a n Z + 3 = 0 \sqrt{3}tan^{3}Z+(2-\sqrt{3})tan^{2}Z-(2+\sqrt{3})tanZ+\sqrt{3}=0

From here, it is very easy to observe that t a n Z = 1 tanZ=1 satisfies the equation.

Thus, by good-ol' factorisation again and again, we get :

( t a n Z 1 ) ( t a n Z + 3 ) ( t a n Z 1 3 ) = 0 (tanZ-1)(tanZ+\sqrt{3})(tanZ-\frac{1}{\sqrt{3}})=0

From there, calculation yields the 3 lowest positive values of x x as:

x = 1 0 , 1 5 , 4 0 x=10^{\circ},15^{\circ},40^{\circ}

Our sum = 65 =\boxed{65}

Good solution. However, the problem is not horrible-looking; it's beautiful

Hobart Pao - 6 years, 7 months ago

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the problem is nice, but it is undeniably horrible looking. I mean.. c'mon....there cannot be any living person who would like to see so many square roots in a single cubic equation whose all terms had not yet been converted to the same variable..


cannot deny that the problem, even though may look tiring, was actually fun :D

Aritra Jana - 6 years, 7 months ago

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