Definition df-cos 11357

Description: Define the cosine function. (Contributed by NM, 14-Mar-2005.)

Assertion

Ref	Expression
df-cos	⊢ cos = (𝑥 ∈ ℂ ↦ (((exp‘(i · 𝑥)) + (exp‘(-i · 𝑥))) / 2))

Detailed syntax breakdown of Definition df-cos

Step	Hyp	Ref	Expression
1		ccos 11351	. 2 class cos
2		vx	. . 3 setvar 𝑥
3		cc 7618	. . 3 class ℂ
4		ci 7622	. . . . . . 7 class i
5	2	cv 1330	. . . . . . 7 class 𝑥
6		cmul 7625	. . . . . . 7 class ·
7	4, 5, 6	co 5774	. . . . . 6 class (i · 𝑥)
8		ce 11348	. . . . . 6 class exp
9	7, 8	cfv 5123	. . . . 5 class (exp‘(i · 𝑥))
10	4	cneg 7934	. . . . . . 7 class -i
11	10, 5, 6	co 5774	. . . . . 6 class (-i · 𝑥)
12	11, 8	cfv 5123	. . . . 5 class (exp‘(-i · 𝑥))
13		caddc 7623	. . . . 5 class +
14	9, 12, 13	co 5774	. . . 4 class ((exp‘(i · 𝑥)) + (exp‘(-i · 𝑥)))
15		c2 8771	. . . 4 class 2
16		cdiv 8432	. . . 4 class /
17	14, 15, 16	co 5774	. . 3 class (((exp‘(i · 𝑥)) + (exp‘(-i · 𝑥))) / 2)
18	2, 3, 17	cmpt 3989	. 2 class (𝑥 ∈ ℂ ↦ (((exp‘(i · 𝑥)) + (exp‘(-i · 𝑥))) / 2))
19	1, 18	wceq 1331	1 wff cos = (𝑥 ∈ ℂ ↦ (((exp‘(i · 𝑥)) + (exp‘(-i · 𝑥))) / 2))

This definition is referenced by:  cosval  11410  cosf  11412  coscn  12859