sin2α = 2cosαsinα
sin2α = sin(α + α) = sinαcosα + cosαsinα = 2sinαcosα
余弦二倍角公式有三组表示形式,三组形式等价:
1. cos2α = 2cos^2(α)-1
2. cos2α = 1 ? 2sin^2(a)
3. cos2α = cos^2(a) ? sin^2(a)
cos2α = cos^2(α)- sin^2(α)= 2cos^2(α)- 1 = 1 - 2sin^2(α)
还可以变形为
sin^2α = (1 - cos2α) /2,
cos^2α =(1 + cos2α)/2
tan2α = 2tanα/[1 - (tanα)^2]
tan(1/2*α)=(sin α)/(1 + cos α) = (1 - cos α)/sin α
tan(2a) = tan(a + a) = (tan(a) + tan(a))/(1 - tan(a)*tan(a) )= 2tanα/[1 - tan^2(a)]
1.求证tan(α/2)=sinα/(1+cosα)=(1 - cosα)/sinα
tan(α/2)
=sin(α/2) / cos(α/2)
=sin²(α/2) / sin(α/2)cos(α/2)
=2sin²(α/2) / 2sin(α/2)cos(α/2)
=2sin²(α/2) / sinα 【正弦二倍角公式】
=[2 - 2cos²(α/2)] / sinα
=[1 + 1-2cos²(α/2)] / sinα
=(1 - cosα) / sinα 【余弦二倍角公式】
=(1 - cosα)(1 + cosα) / sinα(1 + cosα)
=sin²α / sinα(1 + cosα)
=sinα / (1 + cosα)
(降幂公式又称半角公式)
cos^2(α)= [1 + cos2α]/2
sin^2(α)= [1 - cos2α]/2
tan^2(α)= [1- cos2α]/[1 + cos2α]
(sinα + cosα)^2 = 1 + 2sinacosα = 1 + sin2α
(sinα - cosα)^2 = 1 - 2sinαcosα = 1 - sin2α
sin2α = sin^2(α + π/4) - cos^2(α + π/4) = 2sin^2(a + π/4) - 1 = 1 - 2cos^2(α + π/4);
cos2α = 2sin(α + π/4)cos(α + π/4)
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