Question

3 задание нужно срочно

Answer 1

$dfrac{2(cos a+cos3a)}{2sin2a+sin4a} =dfrac{2cdot2cosfrac{a+3a}{2}cosfrac{a-3a}{2}}{2sin2a+2sin2acos2a} =dfrac{4cos2acos(-a)}{2sin2a(1+cos2a)} =\\=dfrac{2cos2acos a}{sin2a(1+cos2a)} =dfrac{2cos2acos a}{sin2acdot2cos^2a} =dfrac{cos2a}{sin2acdotcos a} =dfrac{mathrm{ctg}2a}{cos a}$