00:01
Okay, we are told sine a is negative 4 over 5 and a is in the fourth quadrant, as shown here.
00:07
And the first thing i want to do is find this angle, alpha, the reference angle, but i'll find it to deal with it.
00:17
Four line down here is a right angle.
00:20
And i can see that here we have sine of a is negative sign of alpha, which means negative 4 .4.
00:34
Or fifths is negative sine alpha.
00:37
So we know sine alpha is 4 over 5.
00:43
Here is 4.
00:45
Here is 5.
00:46
O over h, which means my third side has to be 3.
00:51
It's a 3, 4, 5 triangle on pythagoras.
00:56
Now i want to work out, sine 2a first.
01:00
Well, sine 2a is twice sine a cos a double angle formula.
01:08
That you will find on all formula sheets...
