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# C4 Trig help watch

1. Prove that 4(sin4x+cos4x) = cos4x+3

Thanks
2. (Original post by Stokeesy)
Prove that 4(sin4x+cos4x) = cos4x+3

Thanks
Writing cos2x and sin2x in terms of cos2x would be a good starting point.
3. (Original post by Mr Inquisitive)
Writing cos2x and sin2x in terms of cos2x would be a good starting point.
Not really got any where
4. (Original post by Stokeesy)
Not really got any where
What have you tried? Can you show your working out?
5. (Original post by Mr Inquisitive)
What have you tried? Can you show your working out?
4(sin4x+cos4x)
= 4sin4x+4cos4x
= 2sin2x2sin2x+2cos2x2cos2x
= (cos2x-1)(cos2x-1)+(cos2x+1)(cos2x+1)
......
6. (Original post by Stokeesy)
4(sin4x+cos4x)
= 4sin4x+4cos4x
= 2sin2x2sin+2cos2x2cos2?x
= (cos2x-1)(cos2x-1)+(cos2x+1)(cos2x+1)
......
Try starting off by considering
7. (Original post by Stokeesy)
4(sin4x+cos4x)
= 4sin4x+4cos4x
= 2sin2x2sin+2cos2x2cos2?x
= (cos2x-1)(cos2x-1)+(cos2x+1)(cos2x+1)
......
That method's quite messy, and seems to be riddled with mistakes.

Take it one step at a time. First write sin2x and cos2x in terms of cos2x.

We know that cos2x = cos2x - sin2x, so we can make deductions from that as to what they are.

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