Question

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Answer 1

Ответ:

$\frac{sina+sin3a}{sin2a}\cdot \frac{cosa+cos3a}{cos2a}-2=\frac{2\, sin2a\cdot cosa}{sin2a}\cdot \frac{2\, cos2a\cdot cosa}{cos2a}-2=\\\\=2\cdot cosa\cdot 2\cdot cosa-2=2\cdot (2cos^2a-1)=2\cdot (2cos^2a-(sin^2a+cos^2a))=\\\\=2\cdot (cos^2a-sin^2a)=2\cdot cos2a$