Evaluate the exact value of the summation given below, hence determine the sum of the numerator and denominator of the exact value.
Note: This problem appeared in Euclid 2014.
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θ = 1 ∘ ∑ 8 9 ∘ sin 6 θ = θ = 1 ∘ ∑ 4 4 ∘ sin 6 θ + sin 6 4 5 ∘ + θ = 4 6 ∘ ∑ 8 9 ∘ sin 6 θ
Since sin θ = cos ( 9 0 ∘ − θ ) and sin 4 5 ∘ = 2 1 , therefore: θ = 1 ∘ ∑ 4 4 ∘ sin 6 θ + sin 6 4 5 ∘ + θ = 4 6 ∘ ∑ 8 9 ∘ sin 6 θ = θ = 1 ∘ ∑ 4 4 ∘ ( sin 6 θ + cos 6 θ ) + 2 3 1 Now, we need to simplify the summation above. Let s = sin 2 θ and c = cos 2 θ . Hence, by identity, s + c = 1 and 4 c s = sin 2 2 θ . So, θ = 1 ∘ ∑ 4 4 ∘ ( s 3 + c 3 ) = θ = 1 ∘ ∑ 4 4 ∘ ( ( s + c ) ( s 2 − s c + c 2 ) = θ = 1 ∘ ∑ 4 4 ∘ ( 1 ) ( ( s + c ) − 3 s c ) = θ = 1 ∘ ∑ 4 4 ∘ ( 1 − 3 s c ) = θ = 1 ∘ ∑ 4 4 ∘ ( 1 − 4 3 ( 4 s c ) ) = θ = 1 ∘ ∑ 4 4 ∘ ( 1 − 4 3 sin 2 2 θ ) ⇒ 4 4 − 4 3 θ = 1 ∘ ∑ 4 4 ∘ sin 2 2 θ Applying sin θ = cos ( 9 0 ∘ − θ ) to the summation again yields: θ = 1 ∘ ∑ 4 4 ∘ sin 2 2 θ = θ = 1 ∘ ∑ 2 2 ∘ sin 2 2 θ + θ = 2 3 ∘ ∑ 4 4 ∘ sin 2 2 θ = θ = 1 ∘ ∑ 2 2 ∘ sin 2 2 θ + θ = 1 ∘ ∑ 2 2 ∘ cos 2 2 θ ⇒ θ = 1 ∘ ∑ 2 2 ∘ ( sin 2 2 θ + cos 2 2 θ ) = 2 2 With that, we can compute θ = 1 ∘ ∑ 8 9 ∘ sin 6 θ to be: θ = 1 ∘ ∑ 8 9 ∘ sin 6 θ = 4 4 − 4 3 ( 2 2 ) + 2 3 1 = 8 2 2 1 The desired answer is 2 2 1 + 8 = 2 2 9 .