sin 2 ( 5 ∘ ) + sin 2 ( 1 0 ∘ ) + sin 2 ( 1 5 ∘ ) + . . . + sin 2 ( 9 0 ∘ ) = ?
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S = sin 2 5 ∘ + sin 2 1 0 ∘ + sin 2 1 5 ∘ + ⋯ + sin 2 8 0 ∘ + sin 2 8 5 ∘ + sin 2 9 0 ∘ = ( sin 2 5 ∘ + sin 2 8 5 ∘ ) + ( sin 2 1 0 ∘ + sin 2 8 0 ∘ ) + ( sin 2 1 5 ∘ + sin 2 7 5 ∘ ) + ⋯ + ( sin 2 4 0 ∘ + sin 2 5 0 ∘ ) + sin 2 4 5 ∘ + sin 2 9 0 ∘ = ( sin 2 5 ∘ + cos 2 5 ∘ ) + ( sin 2 1 0 ∘ + cos 2 1 0 ∘ ) + ( sin 2 1 5 ∘ + cos 2 1 5 ∘ ) + ⋯ + ( sin 2 4 0 ∘ + cos 2 4 0 ∘ ) + sin 2 4 5 ∘ + sin 2 9 0 ∘ = 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 2 1 + 1 = 9 . 5
@Aly Ahmed , why don't you use LaTex. It is not that difficult. Like a professional use only three dots (...) will do. Not like a kid (.................) \ [ \sin^2 (5^\circ)+\sin^2 (10^\circ)+ \sin^2(15^\circ)+...+\sin^2(90^\circ) = \ ? \ ]
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There are 1 8 terms. The last one is 1 . The middle one, sin 2 ( 4 5 ∘ ) , is 0 . 5 . Let S be the sum of the remaining 1 6 terms. Then S = ( sin 2 ( 5 ∘ ) + sin 2 ( 8 5 ∘ ) ) + ( sin 2 ( 1 0 ∘ ) + sin 2 ( 8 0 ∘ ) ) + ⋯ + ( sin 2 ( 4 0 ∘ ) + sin 2 ( 5 0 ∘ ) ) = ( sin 2 ( 5 ∘ ) + cos 2 ( 5 ∘ ) ) + ( sin 2 ( 1 0 ∘ ) + cos 2 ( 1 0 ∘ ) ) + ⋯ + ( sin 2 ( 4 0 ∘ ) + cos 2 ( 4 0 ∘ ) ) = 1 + 1 + ⋯ + 1 = 8 . So the answer is 1 + 0 . 5 + 8 = 9 . 5 .