2015 Trig

Geometry Level 4

In A B C \bigtriangleup ABC , the opposite sides of A \angle A , B \angle B , C \angle C are a a , b b , c c respectively. If a 2 + b 2 = t c 2 a^{2}+b^{2}=tc^{2} , and cot C = 2015 ( cot A + cot B ) \cot C=2015( \cot A+\cot B) , find the value of the constant t t .


The answer is 4031.0.

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1 solution

Jessica Wang
Jul 11, 2015

cot C = 2015 ( cot A + cot B ) cot C cot A + cot B = 2015 ( cos C sin C ) ( cos A sin A + cos B sin B ) = 2015 \cot C=2015 (\cot A+\cot B) \\ \Rightarrow \frac {\cot C} {\cot A+\cot B} =2015 \\ \Rightarrow \frac{\left ( \frac{\cos C}{\sin C} \right )}{\left ( \frac{\cos A}{\sin A}+\frac{\cos B}{\sin B} \right )}=2015

cos C sin C cos A sin A + cos B sin B = 2015 \Rightarrow \frac{\frac{\cos C}{\sin C}}{\frac{\cos A}{\sin A}+\frac{\cos B}{\sin B}}=2015

cos C sin C × sin A sin B sin ( A + B ) = 2015 \Rightarrow \frac{\cos C}{\sin C}\times \frac{\sin A\cdot \sin B}{\sin (A+B)}=2015

sin A sin B cos C sin 2 C = 2015 \Rightarrow \frac{\sin A\cdot \sin B\cdot \cos C}{\sin^{2}C}=2015

a b c 2 a 2 + b 2 c 2 2 a b = 2015 \Rightarrow \frac{ab}{c^{2}}\cdot \frac{a^{2}+b^{2}-c^{2}}{2ab}=2015

(note that the above step is due to a sin A = b sin B = c sin C \frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C} )

a 2 + b 2 c 2 2 c 2 = 2015 \Rightarrow \frac{a^{2}+b^{2}-c^{2}}{2c^{2}}=2015

t 1 2 = 2015 \Rightarrow \frac{t-1}{2}=2015

t = 4031 . \Rightarrow \boxed{t=4031}.

Moderator note:

Nice trig identity.

What caused you to look into cot C = T ( cot A + cot B ) \cot C = T ( \cot A + \cot B ) ?

(In response to Challenge Master:) The question gives information about relationship among the sides of a triangle, so I thought we may use trig identities related to the sides. They include the sine and cosine rules. The "cotangent equation" in the question might be able to be simplified, and using those identities, be represented in a,b,c form.

Jessica Wang - 5 years, 11 months ago

Help -- how can I make the numbers&words larger? @_@

Jessica Wang - 5 years, 11 months ago

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Don't use \mathrm.

Use \cot, \cos, \sin etc for the trigonometric functions. I've edited your first line as an example.

Calvin Lin Staff - 5 years, 11 months ago

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@Calvin Lin Thank you, I have edited! But it is still tiny when inserting fractions, what should I do?

Jessica Wang - 5 years, 11 months ago

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@Jessica Wang The reason why it looks so small, is that you are using "inline" brackets, which shrinks the equations to make them appear inline with a line of text, whereas the formatting of your equations makes all of them "newline", where we don't want the equations to be shrunk.

I've edited the first 3 equations, to give an example of how this works. If you are familiar with \begin{array}, you can use that the improve the formatting of the code.

Calvin Lin Staff - 5 years, 11 months ago

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