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The following trig identity looks like a mistake but is correct: sin(x + y) sin(x – y) = (sin(x) + sin(y)) (sin(x) – sin(y)) It looks as if someone fallaciously expanded sin(x + y) “=” sin(x) + sin(y) and sin(x – y) “=” sin(x) – sin(y). Although both expansions are wrong, their product is correct. […] The post A unique property of sine first appeared on John D. Cook....