π/2<α<π
cosα = - 0,6
sinα = √1-cos²α=√1-9/25=√16/25 = 0,8
ctg 2α = cos 2α/sin 2a
sin 2α = 2sinα*cosα = 2*0,8*(-0,6)= - 0,96
cos 2a = √1-sin²2α = √1-0,9216 = - 0,28 (π<2α<3π/2)
ctg 2α = cos 2α/sin 2a = - 0,28/-0,96 = 7/24 ≈ 0,2917
По формуле двойного угла
ctg 2a = (ctg²a - 1)/(2ctg a)
ctg a = cos a/sin a = cos a/√(1-cos² a) = (-3/5)/√(16/25) = (-3/5)/(4/5) = -3/4
ctg 2a = [(-3/4)²-1]/[2*(-3/4)] = (9/16-1)/(-6/4) = (-7/16)*(-2/3) = 7/24
ответ: ctg 2a = 7/24
π/2<α<π
cosα = - 0,6
sinα = √1-cos²α=√1-9/25=√16/25 = 0,8
ctg 2α = cos 2α/sin 2a
sin 2α = 2sinα*cosα = 2*0,8*(-0,6)= - 0,96
cos 2a = √1-sin²2α = √1-0,9216 = - 0,28 (π<2α<3π/2)
ctg 2α = cos 2α/sin 2a = - 0,28/-0,96 = 7/24 ≈ 0,2917
По формуле двойного угла
ctg 2a = (ctg²a - 1)/(2ctg a)
ctg a = cos a/sin a = cos a/√(1-cos² a) = (-3/5)/√(16/25) = (-3/5)/(4/5) = -3/4
ctg 2a = [(-3/4)²-1]/[2*(-3/4)] = (9/16-1)/(-6/4) = (-7/16)*(-2/3) = 7/24
ответ: ctg 2a = 7/24