00:01
Ok, let's begin off then with this one here.
00:03
Sine a is minus 3 fifths in quadrant number 3.
00:07
Let's draw the angle.
00:08
Here are the four quadrants.
00:11
The third quadrant is over here.
00:14
So this is the angle a.
00:18
And reference angle, we'll call it alpha, is down here.
00:25
So we have the sine of a is negative sine of alpha.
00:32
And sine a, we're told, is minus 3 fifths.
00:35
So minus 3 over 5 is minus sine alpha.
00:41
Well, this triangle here is right angled, of course.
00:47
And sine of alpha, here is 3, here is 5, over h for sine.
00:55
Missing side is, of course, 4, 3, 4, 5 triangle.
00:59
And we have here sine alpha 3 fifths.
01:06
Now, i want to work out the value of cosine a.
01:15
Cosine of a will have the same value as negative cosine of alpha.
01:23
In that quadrant, cosine is negative.
01:28
And cosine alpha, i can see quite clearly, is 4 over 5 a over h.
01:34
So cosine a, then, is negative 4 fifths.
01:37
That one we'll do in a moment.
01:40
Next, i'll deal with this one.
01:42
Cosine b is minus 3 quarters...