Theorem acoscosb

Description: Relationship between cosine and arccosine. (Contributed by Mario Carneiro, 2-Apr-2015)

	Ref	Expression
Assertion	acoscosb	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( ( arccos ‘ 𝐴 ) = 𝐵 ↔ ( cos ‘ 𝐵 ) = 𝐴 ) )

Proof

Step	Hyp	Ref	Expression
1		cosacos	⊢ ( 𝐴 ∈ ℂ → ( cos ‘ ( arccos ‘ 𝐴 ) ) = 𝐴 )
2	1	3ad2ant1	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( cos ‘ ( arccos ‘ 𝐴 ) ) = 𝐴 )
3		fveqeq2	⊢ ( ( arccos ‘ 𝐴 ) = 𝐵 → ( ( cos ‘ ( arccos ‘ 𝐴 ) ) = 𝐴 ↔ ( cos ‘ 𝐵 ) = 𝐴 ) )
4	2 3	syl5ibcom	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( ( arccos ‘ 𝐴 ) = 𝐵 → ( cos ‘ 𝐵 ) = 𝐴 ) )
5		acoscos	⊢ ( ( 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( arccos ‘ ( cos ‘ 𝐵 ) ) = 𝐵 )
6	5	3adant1	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( arccos ‘ ( cos ‘ 𝐵 ) ) = 𝐵 )
7		fveqeq2	⊢ ( ( cos ‘ 𝐵 ) = 𝐴 → ( ( arccos ‘ ( cos ‘ 𝐵 ) ) = 𝐵 ↔ ( arccos ‘ 𝐴 ) = 𝐵 ) )
8	6 7	syl5ibcom	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( ( cos ‘ 𝐵 ) = 𝐴 → ( arccos ‘ 𝐴 ) = 𝐵 ) )
9	4 8	impbid	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ ( ℜ ‘ 𝐵 ) ∈ ( 0 (,) π ) ) → ( ( arccos ‘ 𝐴 ) = 𝐵 ↔ ( cos ‘ 𝐵 ) = 𝐴 ) )