Consider the function from the positive integers to the reals f ( n ) = sin 1 ∘ + sin 2 ∘ + sin 3 ∘ + ⋯ + sin n ∘ . Find the maximum possible value of f ( n ) as n ranges over all positive integers. Leave your answer to the nearest integer.
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Nice solution @Victor Loh
N O T E 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 ° ) to s i n ( n ° ) First it must be noted that the maximum occurs when n = 1 8 0 . So ∫ 0 π s i n ( x ) d x = 2 This is the area of the graph from 0 to 1 8 0 ° 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 . Note this is only an estimate of the average of all of the 1 8 0 terms. Therefore the total e s t i m a t e d value of the sum of all the terms is π 2 × 1 8 0 = 1 1 4 . 5 9 1 5 5 9 0 2 6 So, rounding off to the nearest integer makes the answer 1 1 5
Pretty close to the real answer: 1 1 4 . 5 8 8 6 5 0 1
general formula is ∑ k = 0 n − 1 s i n ( a + k d ) = s i n ( 2 d ) s i n ( 2 n d ) s i n ( a + 2 ( n − 1 ) d ) here we put a=d=1 and n=180 to obtain the sum to s i n ( 1 7 9 o ) ( s i n ( 1 8 0 o ) can be excluded because it is equal to 0)
did the same!
# 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.
Trigonometric identities are too hard! Let ω = e ( i ⋅ 1 8 0 π ) = c i s ( 1 8 0 π ) then we are interested in I m ( ω 1 + ω 2 + … + ω 1 8 0 ) . Luckily, we can calculate the sum of geometric series, so: ω 1 + ω 2 + … + ω 1 8 0 = ω ( 1 − ω 1 − ω 1 8 0 ) = 1 − ω 2 ω and with enough will (or with Wolfram) you can calculate this number, and take its imaginary part.
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We shall consider the graph of the sin function.
Since the values of sin t ∘ from 1 8 0 ∘ < t ∘ < 3 6 0 ∘ are negative, sin t ∘ = sin ( 1 8 0 − t ) ∘ for all t such that 0 ∘ ≤ t ∘ ≤ 1 8 0 ∘ and the period of the sin function is 3 6 0 ∘ , it suffices to use n = 1 8 0 .
Now, we want to find a formula for sin t ∘ + sin ( 2 t ) ∘ + ⋯ + sin ( n t ) ∘ . We proceed to multiply and divide the expression by 2 sin ( 2 t ) ∘ to obtain
2 sin ( 2 t ) ∘ 2 sin ( 2 t ) ∘ [ sin t ∘ + sin ( 2 t ) ∘ + ⋯ + sin ( n t ) ∘ ]
We next distribute the 2 sin ( 2 t ) ∘ to all the terms in the numerator. We also know that 2 sin x ∘ sin y ∘ = cos ( x − y ) ∘ − cos ( x + y ) ∘ . Hence, by letting x = 2 t and y = t , we obtain a telescopic sum which cancels out to be
sin t ∘ + sin ( 2 t ) ∘ + ⋯ + sin ( n t ) ∘ = 2 sin ( 2 t ) ∘ cos ( 2 t ) ∘ − cos ( 2 ( 2 n − 1 ) t ) ∘
By letting t = 1 and n = 1 8 0 , we have our desired answer to be 1 1 5 , and we are done. □