Product of Sum of Tangents?

Geometry Level 4

Find the value of x x such that 2 x = n = 1 29 ( 3 + tan ( n ) ) \displaystyle 2^x= \prod^{29}_{n=1}(\sqrt3 + \tan (n^{\circ})) .


The answer is 29.

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3 solutions

Pedro Arantes
Nov 29, 2015

Let f ( n ) = 3 + tan ( n ) f(n) = \sqrt3 + \tan(n^\circ) .

We know that f ( n ) f(n) can be written by:

f ( n ) = 1 tan ( 3 0 ) + tan ( n ) = cos ( 3 0 ) sin ( 3 0 ) + sin ( n ) cos ( n ) = cos ( n ) cos ( 3 0 ) + sin ( n ) sin ( 3 0 ) cos ( n ) sin ( 3 0 ) = cos ( 3 0 n ) cos ( n ) 1 sin ( 3 0 ) f(n) = \dfrac{1}{\tan(30^\circ)} + \tan(n^\circ) = \dfrac{\cos(30^\circ)}{\sin(30^\circ)} + \frac{\sin(n^\circ)}{\cos(n^\circ)} = \dfrac{\cos(n^\circ)\cos(30^\circ) + \sin(n^\circ)\sin(30^\circ)}{\cos(n^\circ)\sin(30^\circ)} = \dfrac{\cos(30^\circ - n^\circ)}{\cos(n^\circ)} \dfrac{1}{\sin(30^\circ)} .

Therefore, n = 1 29 f ( n ) = 1 sin 29 ( 3 0 ) cos ( 2 9 ) cos ( 2 8 ) cos ( 2 7 ) . . . cos ( 1 ) cos ( 1 ) cos ( 2 ) cos ( 3 ) . . . cos ( 2 9 ) = 2 29 . \displaystyle \prod^{29}_{n=1} f(n) = \dfrac{1}{\sin^{29}(30^\circ)} \dfrac{\cos(29^\circ)\cos(28^\circ)\cos(27^\circ)...\cos(1^\circ)}{\cos(1^\circ)\cos(2^\circ)\cos(3^\circ)...\cos(29^\circ)} = 2^{29}.

Thus, x = 29 x = \boxed{29}

why you would use the square root of 3 as the inverse of the tan(30º) if you can use directly as the tan(60º) ?

Vitor Santos - 5 years, 6 months ago

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Because, in my resolution, I've used cotg(30) to represent sqrt3. But I don't know the code to write "cotg" here (do you know?), so I used the inverse of tan(30). Anyway, I didn't realize that using tg(60) works too, thank you!

Pedro Arantes - 5 years, 6 months ago

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In LaTeX editor is \cot {x} that should appear as cot x \cot {x} and you're welcome.

Vitor Santos - 5 years, 6 months ago
Aakash Khandelwal
Nov 30, 2015

The above expression can be : 2sin(60+x)/cos(x) put consecutively the values and you get x=29.

Kaustubh Miglani
Nov 28, 2015

let α=(3√+tan(1)).(3√+tan(2)).(3√+tan(3))......(3√+tan(29)) Which can be written as α=(3√+tan(1)).(3√+tan(29)).(3√+tan(2)).(3√+tan(28)).......(3√+tan(14)).(3√+tan(16)).(3√+tan(15))

Now,, notice that (3√+tan(1)).(3√+tan(29))=3+3√(tan(1)+tan(29))+tan(1).tan(29)...........(1)

Now we know from the formula that tan(30)=tan(1)+tan(29)1−tan(1)tan(29) Therefore, tan(1)+tan(29)=13√(1−tan(1)tan(29)) , so putting this value in (1), we get (3√+tan(1)).(3√+tan(29))=3+(1−tan(1).tan(29))+tan(1).tan(29)=4

So, like this, all the pairs cancel out to give 4 in each bracket, So, the original problem becomes α=414((3√+tan(15))

Now, we see that (3√+tan(15))=3√+sin(30)1+cos(30)=2

So, the original number α is nothing but, α=(414).2

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