00:01
Okay, we're told sine theta is 1 over 9 and in the second quadrant.
00:06
So that means theta has to be this angle here.
00:14
And here is alpha by reference angle.
00:17
So i know then that sine theta has the same value as sine alpha.
00:28
Because in that quadrant, sine is plus.
00:31
So if i draw a line down here, i know sine alpha.
00:37
Alpha then is 1 over 9.
00:43
Here is 1 and here is 9.
00:46
0 over h, which means the missing side down here has to be the root of 9 squared minus 1 squared, so root of 80, 81 minus 1.
01:02
So now i can work out cosine theta.
01:08
Cosine theta is negative cosine alpha...