sinπ/10+sin13π/10=-1/2
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sinπ/10×sin13π/10
=-sinπ/10×sin3π/10
=-sinπ/10×Cos2π/10
=-sinπ/10×[1-2(Sinπ/10)^2]
=-(V5-1)/4 [1-2[(V5-1)/4]^2]
=-(V5-1)/4 * 4(V5+1)/16
=-1/4
Sin 18=sin π/10 = (V5-1)/4
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