Trigonometry I

Geometry Level 3

If s i n x + c o s x = 3 + 1 2 \displaystyle sin x + cos x = \frac{\sqrt{3} + 1}{2}

tan x + cot x = a + b c \displaystyle \tan x + \cot x = \frac{\sqrt{a} + b}{c}

Find a + b + c \displaystyle a+b+c


The answer is 51.

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4 solutions

Gabriel Merces
Apr 3, 2014

We Have tan x + cot x = sin x cos x + cos x sin x = sin 2 x + cos 2 x sin x cos x = 1 sin x cos x \displaystyle \tan x + \cot x = \frac{\sin x}{\cos x} + \frac{\cos x}{\sin x} = \frac{\sin^2 x + \cos^2 x}{\sin x\cos x} = \frac{1}{\sin x\cos x}

So We Want to Know How Much Is sin x c o s x \displaystyle \sin x cosx

Squaring the Equality You Know, We Have:

( sin x + cos x ) 2 = ( 3 + 1 2 ) 2 sin 2 x + 2 sin x cos x + cos 2 x = 4 + 2 3 4 1 + 2 sin x cos x = 1 + 3 2 2 sin x cos x = 3 2 sin x cos x = 3 4 tan x + cot x = 1 sin x cos x = 4 3 3 48 3 . \begin{aligned} (\sin x + \cos x)^2 &= \left(\frac{\sqrt{3}+1}{2}\right)^2\\ \sin^2 x + 2\sin x\cos x + \cos^2 x & = \frac{4 + 2\sqrt{3}}{4}\\ 1 + 2\sin x \cos x &= 1 + \frac{\sqrt{3}}{2}\\ 2\sin x \cos x &= \frac{\sqrt{3}}{2}\\ \sin x \cos x &= \frac{\sqrt{3}}{4}\\ \tan x + \cot x = \frac{1}{\sin x\cos x} &= \frac{4\sqrt{3}}{3} \rightarrow \frac{\sqrt{48}}{3} . \end{aligned}

\displaystyle a=48 &there4 b=0 &there4 c=3

a + b + c = 48 + 0 + 3 = 51 \displaystyle a+b+c = 48+0+3 = \boxed{51}

Dude, I solved this equation, and I got 4 3 3 \frac{4\sqrt{3}}{3} , and I just spent half an hour trying to manipulate that into the form given. Please specify a better form and change the answer.

Finn Hulse - 7 years, 2 months ago

Yesterday, I had typed 51 and it was shown as wrong answer. Today, I typed 51 and my answer came out right!!

Shabarish Ch - 7 years, 2 months ago

By a nice guess, I got x=30 degrees, also you should've specified that a may not be square free, because in most questions it is square free and all are positive.

Satvik Golechha - 7 years, 2 months ago
Cody Johnson
Apr 4, 2014

sin x + cos x = 3 + 1 2 = 3 2 + 1 2 \sin x+\cos x=\frac{\sqrt3+1}2=\frac{\sqrt3}2+\frac12

But I know a value of x x such that sin x = 3 2 \sin x=\frac{\sqrt3}2 and cos x = 1 2 \cos x=\frac12 . Hm, I forget that value of x x , oh well. Hm, I also forget if it was tan x \tan x or cot x \cot x that was sin x cos x \frac{\sin x}{\cos x} , but I remember that cot x = 1 tan x \cot x=\frac1{\tan x} . No worries, I know that one of tan x \tan x or cot x \cot x is 3 \sqrt3 and the other is 3 3 \frac{\sqrt3}3 . But wait, that doesn't matter since we're adding them! tan x + cot x = 4 3 3 \tan x+\cot x=\frac{4\sqrt3}3 . Whew, excuse my ignorance.

Curtis Clement
Feb 14, 2015

The equation t a n ( x ) + c o t ( x ) \displaystyle\ tan(x) + cot(x) is symmetrical in sine and cosine so ,we can assign them individual values, without loss of generality. Let s i n x = 3 2 , c o s x = 1 2 \displaystyle\ sinx = \frac{\sqrt{3}}{2} \ , \ cosx = \frac{1}{2} . This leaves t a n x + c o t x = 3 + 1 3 = 4 3 = 48 3 a + b + c = 51 tanx + cotx = \sqrt{3} + \frac{1}{\sqrt{3}} = \frac{4}{\sqrt{3}} = \frac{\sqrt{48}}{3}\therefore\ a+b+c = \boxed{51}

Puzzle Seeker
May 24, 2014

sin x + cos x = (root 3 + 1)/2 = (root 3)/2 +1/2, so if sin x = (root 3)/2 and cos x = 1/2, so x can be taken as being equal to 60 degrees. Then you can substitute the value of x in tan x + cot x and than solve it.

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