Sine and sines and sines and

Geometry Level 5

Consider the function from the positive integers to the reals f ( n ) = sin 1 + sin 2 + sin 3 + + sin n f(n)=\sin 1^{\circ}+\sin 2^{\circ}+ \sin 3^{\circ}+\cdots+\sin n^{\circ} . Find the maximum possible value of f ( n ) f(n) as n n ranges over all positive integers. Leave your answer to the nearest integer.


The answer is 115.

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5 solutions

Victor Loh
Aug 23, 2014

We shall consider the graph of the sin \sin function.

Since the values of sin t \sin t^{\circ} from 18 0 < t < 36 0 180^{\circ}<t^{\circ}<360^{\circ} are negative, sin t = sin ( 180 t ) \sin t^{\circ} = \sin(180-t)^{\circ} for all t t such that 0 t 18 0 0^{\circ} \leq t^{\circ} \leq 180^{\circ} and the period of the sin \sin function is 36 0 360^{\circ} , it suffices to use n = 180 n=180 .

Now, we want to find a formula for sin t + sin ( 2 t ) + + sin ( n t ) \sin t^{\circ}+\sin (2t)^{\circ}+\cdots+\sin (nt)^{\circ} . We proceed to multiply and divide the expression by 2 sin ( t 2 ) 2\sin\left(\frac{t}{2}\right)^{\circ} to obtain

2 sin ( t 2 ) [ sin t + sin ( 2 t ) + + sin ( n t ) ] 2 sin ( t 2 ) \frac{2\sin\left(\frac{t}{2}\right)^{\circ}\left[\sin t^{\circ}+\sin (2t)^{\circ}+\cdots+\sin (nt)^{\circ}\right]}{2\sin\left(\frac{t}{2}\right)^{\circ}}

We next distribute the 2 sin ( t 2 ) 2\sin\left(\frac{t}{2}\right)^{\circ} to all the terms in the numerator. We also know that 2 sin x sin y = cos ( x y ) cos ( x + y ) 2\sin x^{\circ}\sin y^{\circ}=\cos(x-y)^{\circ}-\cos(x+y)^{\circ} . Hence, by letting x = t 2 x=\frac{t}{2} and y = t y=t , we obtain a telescopic sum which cancels out to be

sin t + sin ( 2 t ) + + sin ( n t ) = cos ( t 2 ) cos ( ( 2 n 1 ) t 2 ) 2 sin ( t 2 ) \sin t^{\circ}+\sin (2t)^{\circ}+\cdots+\sin (nt)^{\circ}=\frac{\cos\left(\frac{t}{2}\right)^{\circ}-\cos\left(\frac{(2n-1)t}{2}\right)^{\circ}}{2\sin\left(\frac{t}{2}\right)^{\circ}}

By letting t = 1 t=1 and n = 180 n=180 , we have our desired answer to be 115 \boxed{115} , and we are done. \square

Nice solution @Victor Loh

Julian Poon - 6 years, 8 months ago
Julian Poon
Sep 21, 2014

N O T E \bf{NOTE} This is a method that only closely estimates the value. It can be used in this question only because there are 180 terms and thus it would give a reasonably close estimate. Do not use this method if it has very little terms (like 3 or 4)

The idea is to find the average of all the terms from s i n ( 1 ° ) sin(1°) to s i n ( n ° ) sin(n°) First it must be noted that the maximum occurs when n = 180 n=180 . So 0 π s i n ( x ) d x = 2 \int _{ 0 }^{ \pi }{ sin(x)dx=2 } This is the area of the graph from 0 0 to 180 ° 180° Then the average can be found by assuming the area to be of a rectangle, with its base on the x-axis of length π \pi . Note this is only an estimate of the average of all of the 180 180 terms. Therefore the total e s t i m a t e d \bf{estimated} value of the sum of all the terms is 2 × 180 π = 114.591559026 \frac{2\times180}{\pi}=114.591559026 So, rounding off to the nearest integer makes the answer 115 \boxed{115}

Pretty close to the real answer: 114.5886501 114.5886501

general formula is k = 0 n 1 s i n ( a + k d ) = s i n ( n d 2 ) s i n ( a + ( n 1 ) d 2 ) s i n ( d 2 ) \sum _{ k=0 }^{ n-1 }{ sin(a+kd) } =\frac { sin(\frac { nd }{ 2 } )sin(a+\frac { (n-1)d }{ 2 } ) }{ sin(\frac { d }{ 2 } ) } here we put a=d=1 and n=180 to obtain the sum to s i n ( 179 o ) sin({ 179 }^{ o }) ( s i n ( 180 o ) (sin({ 180 }^{ o }) can be excluded because it is equal to 0)

did the same!

Kartik Sharma - 6 years, 8 months ago
Shreyash S
Aug 25, 2014

I used the python code

# Code by Shreyash
def f(n):
    res=0
    for i in xrange(1,n+1):
        res += math.sin(math.radians(i))
    return res 
math.ceil(max([f(n) for n in xrange(181)]))

Excellent :D Superb use of pYTHon.

Jayakumar Krishnan - 6 years, 8 months ago
Ohad Klein
Sep 11, 2014

Trigonometric identities are too hard! Let ω = e ( i π 180 ) = c i s ( π 180 ) \omega = e^{\left (i\cdot\frac{\pi}{180} \right )}=cis\left ( \frac{\pi}{180} \right ) then we are interested in I m ( ω 1 + ω 2 + + ω 180 ) \mathrm{Im}\left ( \omega ^{1}+\omega ^{2}+\ldots+\omega ^{180} \right ) . Luckily, we can calculate the sum of geometric series, so: ω 1 + ω 2 + + ω 180 = ω ( 1 ω 180 1 ω ) = 2 ω 1 ω \omega ^{1}+\omega ^{2}+\ldots+\omega ^{180}=\omega\left ( \frac{1-\omega^{180}}{1-\omega} \right )=\frac{2\omega}{1-\omega} and with enough will (or with Wolfram) you can calculate this number, and take its imaginary part.

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