三角函数的二倍角公式:
正弦二倍角:sin2α = 2cosαsin
推导:sin2A = sin(A+A) = sinAcosA + cosAsinA = 2sinAcosA
余弦二倍角:余弦二倍角公式有三组表示形式,三组形式等价:1.cos2a = 2cos2α-1
2、cos2α = 1-2sin2 α
3、cos2a=cos2a-sin2a
推导:cos2A = cos(A+A) = cosAcosA - sinAsinA = cos²A- sin²A = 2cos²A - 1=1-2sin²A
正切二倍角:
tan2α = 2tanα/[1 - (tanα)^2]
tan(1/2*α)=(sin α)/(1+cos α)=(1-cos α)/sinα
推导:tan(2a) = tan(a+a) = (tan(a) + tan(a))/(1 - tan(a)*tan(a) )= 2tanα/(1 -tan²α)