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cos2x-cosx=0
cos2x=2cos²x-1
cos2x-cosx=0, 2cos²x-cosx-1=0,
Замена t=cosx, 2t²-t-1=0, D=1+2*4=9, t₁=(1-3)/4=-1/2, t₂=(1+3)/4=1
cosx=-1/2, x=±(π-arccos1/2)+2πn, x=±(π-π/3)+2πn, x=±2π/3+2πn, n∈Z
c0sx=1, x=π+2πk, k∈Z
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cos2x=2cos²x-1
cos2x-cosx=0, 2cos²x-cosx-1=0,
Замена t=cosx, 2t²-t-1=0, D=1+2*4=9, t₁=(1-3)/4=-1/2, t₂=(1+3)/4=1
cosx=-1/2, x=±(π-arccos1/2)+2πn, x=±(π-π/3)+2πn, x=±2π/3+2πn, n∈Z
c0sx=1, x=π+2πk, k∈Z