00:01
Okay, we're to cosine x is negative four -fifths, and we're in the second quadrant between pi by two and pi, between 90 and 180 degrees.
00:10
So let's just draw a diagram then.
00:12
Here is the grid and the four quadrants.
00:16
I know i'm going to be over here in the second quadrant, and the angle x will be there, which means my reference angle, it's called alpha, it will be here.
00:31
Now, i know that cosine x has the same value as negative cosine alpha, which means then cosine alpha has to be the four -fifths.
00:51
I draw a triangle then like this.
00:55
Here is five, here is four, a over h.
00:59
Missing side has to be three.
01:03
Three, four, five triangle from pythagoras.
01:06
Now, i want to work out, first of all, sine of 2x.
01:13
Well, that's twice sine x, cos x, the standard formula on all formula sheets to sine x cos x.
01:23
So, 2...