Trigonometry Tangents

Geometry Level 2

Find the sum, in degrees, of all θ \theta between 0 0^{\circ} and 36 0 360^{\circ} such that

tan ( 2 θ 33 0 ) = 3 . \tan(2\theta-330^{\circ})=\sqrt{3}.


The answer is 600.

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3 solutions

Victor Loh
Jul 13, 2014

We have

0 < θ < 36 0 0^{\circ}<\theta<360^{\circ}

0 < 2 θ < 72 0 \implies 0^{\circ}<2\theta<720^{\circ}

33 0 < 2 θ 33 0 < 39 0 \implies -330^{\circ}<2\theta-330^{\circ}<390^{\circ}

Since tan \tan is only positive when 2 θ 33 0 2\theta-330^{\circ} lies in the first and third quadrant, and tan 6 0 = 3 \tan 60^{\circ}=\sqrt{3} , we have 2 θ 33 0 = 6 0 , 24 0 2\theta-330^{\circ}=60^{\circ}, 240^{\circ} .

Since tan \tan has a periodicity of 18 0 180^{\circ} , the following values of 2 θ 33 0 2\theta-330^{\circ} are possible: 30 0 , 12 0 , 6 0 , 24 0 -300^{\circ}, -120^{\circ}, 60^{\circ}, 240^{\circ} .

Thus θ = 1 5 , 10 5 , 19 5 , 28 5 \theta=15^{\circ}, 105^{\circ}, 195^{\circ}, 285^{\circ} . Summing these up we have 60 0 \boxed{600^{\circ}} and we are done.

The question does not make clear whether the range is inclusive or exclusive, so I included the solution theta = 360, which produces 2*theta - 330 = 390, which also has the correct tangent. This gives a sum of 990, but was marked incorrect

Michael Beck - 5 years, 7 months ago

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390 degrees does not give the right tangent. This is equivalent to 30 degrees, which gives 1/sqrt(3) as its tangent.

Kunal Kantaria - 5 years, 3 months ago

-300 and -120 is the value of 2 theta - 30, not theta itself...

Victor Loh - 6 years, 11 months ago

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hey!! how can you take -300 and -240...i thought negative angle were not allowed!!! its only 60 and 240

Krishna Ramesh - 6 years, 11 months ago

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he said \theta must be in [0,360] whereas we see 2\theta - 330 can take values like -300,-120 which is justified

Akash Baran Ghosh - 6 years, 9 months ago

Cool. I solved the same way!

Muhammad Tariq - 6 years, 8 months ago

You said theta must be between 0 and 360 but you have included -300 and -120!!!!

Krishna Ar - 6 years, 11 months ago

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Well he said θ \theta must be in [0,360] whereas we see 2 θ 330 2\theta-330 assumes -300,-120 which is justified.

Nishant Sharma - 6 years, 11 months ago

Best of all of you other trigonometry questions!!! Keep it up!!! By the way, I also did with the same method.

Kartik Sharma - 6 years, 11 months ago

using notable angles, we know that:

tan ( β ) = 3 \tan (\beta) = \sqrt3 when β = 60 + 180 n \beta = 60 + 180*n deg, (1) w/ n = 0 , ± 1 , ± 2 , ± 3... n = 0,\pm1,\pm2,\pm3...

So, making equal the trigonometric fuction argument with (1):

2 θ 330 = 60 + 180 n \Rightarrow 2\theta -330=60+180n

θ = 195 + 90 n \Rightarrow \theta = 195+90n

which values included in 0 < θ < 360 0 < \theta < 360 deg. are:

θ = 15 , 105 , 195 , 285 \Rightarrow \theta = 15,105,195,285 and suming these numbers we obtain 600 degres

according to trigonometric equations if tan(alpha)=tan(theta) then alpha=n pi+(theta), then 2x-330=n pi+60 where n is integer, solving this for n=0,1,-1,-2 we get alpha=13 pi/12, 19 pi/12, 7 pi/12, pi/12...summing this we 10 pi/3 which is 600`

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