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tan(A/2)=√((1-cosA)/((1+cosA))
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tan(A/2)=-√((1-cosA)/((1+cosA))
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ctg(A/2)=√((1+cosA)/((1-cosA))
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ctg(A/2)=-√((1+cosA)/((1-cosA))
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2sinAcosB=sin(A+B)+sin(A-B)
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2cosAsinB=sin(A+B)-sin(A-B)
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2cosAcosB=cos(A+B)-sin(A-B)
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2cosAcosB=cos(A+B)-sin(A-B) -2sinAsinB=cos(A+B)-cos(A-B)
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sinA+sinB=2sin((A+B)/2)cos((A-B)/2
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cosA+cosB=2cos((A+B)/2)sin((A-B)/2)
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