第1个回答 2019-10-13
正弦二倍角公式: sin2α
=
2cosαsinα
推导:
sin2α
=
sin(α+α)
=
sinαcosα
+
cosαsinα=
2sinαcosα
余弦二倍角公式:
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
α
3.cos2α
=
cos^2
α
−
sin^2
α
推导:
cos2A
=
cos(A+A)
=
cosAcosA
-
sinAsinA
=
cos^2
A-
sin^2
A
=
2cos^2
A
-
1=1
-
2sin^2
A
正切二倍角公式:
tan2α
=
2tanα/[1
-
(tanα)^2]
tan(1/2*α)=(sin
α)/(1+cos
α)=(1-cos
α)/sin
α
推导:
tan(2a)
=
tan(a+a)
=
(tan(a)
+
tan(a))/(1
-
tan(a)*tan(a)
)=
2tanα/[1
-
(tanα)^2]