Step |
Hyp |
Ref |
Expression |
1 |
|
picn |
|- _pi e. CC |
2 |
|
2cn |
|- 2 e. CC |
3 |
|
2ne0 |
|- 2 =/= 0 |
4 |
1 2 3
|
divcli |
|- ( _pi / 2 ) e. CC |
5 |
|
cos2t |
|- ( ( _pi / 2 ) e. CC -> ( cos ` ( 2 x. ( _pi / 2 ) ) ) = ( ( 2 x. ( ( cos ` ( _pi / 2 ) ) ^ 2 ) ) - 1 ) ) |
6 |
4 5
|
ax-mp |
|- ( cos ` ( 2 x. ( _pi / 2 ) ) ) = ( ( 2 x. ( ( cos ` ( _pi / 2 ) ) ^ 2 ) ) - 1 ) |
7 |
1 2 3
|
divcan2i |
|- ( 2 x. ( _pi / 2 ) ) = _pi |
8 |
7
|
fveq2i |
|- ( cos ` ( 2 x. ( _pi / 2 ) ) ) = ( cos ` _pi ) |
9 |
|
coshalfpi |
|- ( cos ` ( _pi / 2 ) ) = 0 |
10 |
9
|
oveq1i |
|- ( ( cos ` ( _pi / 2 ) ) ^ 2 ) = ( 0 ^ 2 ) |
11 |
|
sq0 |
|- ( 0 ^ 2 ) = 0 |
12 |
10 11
|
eqtri |
|- ( ( cos ` ( _pi / 2 ) ) ^ 2 ) = 0 |
13 |
12
|
oveq2i |
|- ( 2 x. ( ( cos ` ( _pi / 2 ) ) ^ 2 ) ) = ( 2 x. 0 ) |
14 |
|
2t0e0 |
|- ( 2 x. 0 ) = 0 |
15 |
13 14
|
eqtri |
|- ( 2 x. ( ( cos ` ( _pi / 2 ) ) ^ 2 ) ) = 0 |
16 |
15
|
oveq1i |
|- ( ( 2 x. ( ( cos ` ( _pi / 2 ) ) ^ 2 ) ) - 1 ) = ( 0 - 1 ) |
17 |
|
df-neg |
|- -u 1 = ( 0 - 1 ) |
18 |
16 17
|
eqtr4i |
|- ( ( 2 x. ( ( cos ` ( _pi / 2 ) ) ^ 2 ) ) - 1 ) = -u 1 |
19 |
6 8 18
|
3eqtr3i |
|- ( cos ` _pi ) = -u 1 |