問28
(1)
左辺
=((cos θ - sin θ)/cos θ + sin θ))^{2}
=(1-2sin θ cos θ)/(1+2sin θ cos θ)
=(1-2sin 2θ)/1+2sin 2θ)
(2)
左辺
=4cos^{3}θ-3cos θ+3sin θ-4^{3}sin θ
=(cos θ - sin θ)(4(cos^{2}+cos θ sin θ + sin^{2}θ) - 3)
=(cos θ - sin θ)(1-2sin θ cos θ)
(3)
左辺
=tan(θ+2θ)
=(tan θ + tan 2θ)/(1-tan θ tan 2θ)
=(tan θ + 2tan θ/(1-tan^{2}θ))/(1-2tan^{2}θ/(1-tan^{2}θ))
=(3tan θ - tan^{3}θ)/(1-tan^{2}θ) * (1-tan^{2}θ)/(1-3tan^{2}θ)
=(3tan θ - tan^{3}θ)/(1-3tan^{2}θ)
(4)
左辺
=(2sin θ/2 cos θ/2 +1-(cos^{2}θ/2-sin^{2}θ/2))/(2sin θ/2 cos θ/2+(cos^{2}θ/2+sin^{2}θ/2))
=(2sin θ/2 cos θ/2+2sin^{2}θ/2)/(2sin θ/2 cos θ/2 +2cos^{2}θ/2)
=sin θ/2 / cos θ/2
=tan θ/2
(5)
左辺
=tan θ/2 -(tan θ/3 + tan θ/6)
=tan θ/2 - (tan(θ/3 + θ/6)(1-tan θ/3 tan θ/6)
=tan θ/2(1-1+tan θ/3 tan θ/6)
=tan θ/2 tan θ/3 tan θ/6
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