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# C3 Trig function question-poss error in book? watch

1. Question is to convert the following expression into a single trig function:

1 + cos 4x

But I got 2cos^2 (2x)

Did this:

cos4x= cos(2x +2x)

= 2cos^2(2x) -1

so 1+ 2cos^2(2x) -1

becomes 2cos^2 (2x)

I don't see why in the book it is just x.

Thanks
2. (Original post by ViolinGirl)
Question is to convert the following expression into a single trig function:

1 + cos 4x

But I got 2cos^2 (2x)

Did this:

cos4x= cos(2x +2x)

= 2cos^2(2x) -1

so 1+ 2cos^2(2x) -1

becomes 2cos^2 (2x)

I don't see why in the book it is just x.

Thanks
yeah you're right

(for future reference, you could check stuff like this yourself by plugging a value for x into the original expression and in your expression, if you get the same answer out you're right )
3. (Original post by Pheylan)
yeah you're right

(for future reference, you could check stuff like this yourself by plugging a value for x into the original expression and in your expression, if you get the same answer out you're right )

Thank you
Why didn't I think of that?

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