1) cos5α-cosα = -2sin2α*sin3α ;
2) cosα - cos3α +2sin2α = 2sinα *sin2α +2sin2α =2sin2α*(sinα+1) =
2sin2α(1 +cos(π/2 -α)) =4sin2α*cos² (π/4 - α/2).
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cosα - cos3α +2sin2α = 2sinα *sin2α +2sin2α =2sin2α*(sinα+1) =
2sin2α(sinα+sinπ/2) =4sin2α*sin(π/4 +α/2)*cos(π/4 -α/2) =
[[ но *** sin(π/4 +α/2) =cos(π/2 -(π/4 +α/2)) =cos(π/4 - α/2) *** ]] =
4sin2α*cos² (π/4 - α/2).