00:01
I went ahead and wrote down the answer so i know where i need to go with this.
00:06
Because if you're starting with cosine of 3x, there's a lot of different ways you can go about doing this problem.
00:12
You know, you could even start with changing 3x to be 2x plus x.
00:19
And then if you look at the trig identities for the sum angle, this is equal to cosine of the first angle, cosine of the second angle.
00:31
I have to double check that it's plus or minus.
00:33
It's minus sign of the first angle, sign of the second angle.
00:41
And i'm not sure if this is even the direction i need to go with the problem because there are three different formulas for cosine of 2x.
00:49
But there's only one formula for this sign, sine of 2x.
00:54
And i'm skimming it and it is 2 sine cosine, 2 sine, cosine x this is what i just underlined several times is equal to this but you still have this sign of x right here and having this answer kind of gives me direction to go with this so may i'll use the formula for cosine of 2x that only involves cosine so this one that only involves cosine is two cosine squared of x let me double check two cosine squared of x minus one so what i just put in parentheses is equal to that.
01:39
So times cosine of x.
01:40
So let me just clean that up real quick.
01:42
So what do i have? let me distribute that cosine in.
01:44
So that's two cosine cubed of x minus cosine of x.
01:51
And you can see how close we are to getting that answer...