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# C4 Integration/Trigonometry Question Watch

1. Hi,

I decided to try a Solomon paper for the first time today and I'm struggling with the question attached.

I know that cos(P-Q) = -2sin((P+Q)/2)sin((P-Q)/2)

and I worked out that P=5x and Q=x

so I thought I would get -cos(5x-x) = -cos4x

I'm confused about how I'm meant to get to cosx - cos5x
2. The attachment keeps disappearing but it's this.
3. (Original post by etherealinsanity)
Hi,

I decided to try a Solomon paper for the first time today and I'm struggling with the question attached.

I know that cos(P-Q) = -2sin((P+Q)/2)sin((P-Q)/2)

and I worked out that P=5x and Q=x

so I thought I would get -cos(5x-x) = -cos4x

I'm confused about how I'm meant to get to cosx - cos5x
You've got the identity wrong. It should be

cos(P) - cos(Q) = -2sin((P+Q)/2)sin((P-Q)/2)
4. (Original post by notnek)
You've got the identity wrong. It should be

cos(P) - cos(Q) = -2sin((P+Q)/2)sin((P-Q)/2)

Oh, okay. Thank you! That makes a lot more sense.

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