Дано cosα= -5/13, π<α<3π/2

Найти cosα/2, ctgα

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cosa=-5/13sina=-√(1-cos²a)=-√(1-25/169)=-√(144/169)=-12/13ctga=cosa/sina=-5/13:(-12/13)=5/13*13/12=5/12cos²a/2=(1+cosa)/2=(1-5/13)/2=8/13:2=4/13cosa/2=+-2/√13