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C3 Trigonometry question

Is 2cos3xcos7x = cos10x +cos4x OR cos10x +cos(-4x) ?

I thought the rule was first angle - 2nd angle, does it not matter ?
Original post by JanaALEVEL
Is 2cos3xcos7x = cos10x +cos4x OR cos10x +cos(-4x) ?

I thought the rule was first angle - 2nd angle, does it not matter ?


The cosine function is symmetrial about x=0, i.e. it's an even function.

cos(4x)=cos(4x)\cos (-4x) = \cos (4x)
Reply 2
Original post by ghostwalker
The cosine function is symmetrial about x=0, i.e. it's an even function.

cos(4x)=cos(4x)\cos (-4x) = \cos (4x)


OH RIGHT.
But if it was 2sin3xcos7x it would equal sin10x +sin(-4x) not sin10x +sin4x right ?
Original post by JanaALEVEL
OH RIGHT.
But if it was 2sin3xcos7x it would equal sin10x +sin(-4x) not sin10x +sin4x right ?


Correct. And also be aware that sine is an "odd" function, i.e. f(-x) = -f(x)

So, sin10x +sin(-4x) is the same as sin10x - sin(4x)

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