00:01
Okay, we got to cosine x is negative three -fifths, the third quadrant, so what the red line is on my diagram.
00:08
And that makes sense because in that quadrant, cosine is minus.
00:12
It's plus in 1 and 4, and it's minus in 2 and 3.
00:16
I want to work out cosine of 2x.
00:20
Well, there are three ways to write cosine 2x, which you can find on all formula sheets.
00:25
We have cos squared x minus sine squared x.
00:29
We have 1 minus 2 sine squared x and we have 2 cos squared x minus 1.
00:39
So three ways to write cos 2x.
00:42
I'll choose the last one.
00:45
So what we have then is 2...