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# P2 question watch

1. Find in ssurd fortm, the exacty value of cos 75 - whats the easiest way to do it?
2. (Original post by TheWolf)
Find in ssurd fortm, the exacty value of cos 75 - whats the easiest way to do it?
i would do cos 75 = cos(30+45),
now use the identity cos(A+B) = cosAcosB - sinAsinB
3. (Original post by TheWolf)
Find in ssurd fortm, the exacty value of cos 75 - whats the easiest way to do it?
Cos(45 + 30) = Cos45Cos30 - Sin45Sin30.

You should know the values of these as they're all standard forms.
4. (Original post by TheWolf)
Find in ssurd fortm, the exacty value of cos 75 - whats the easiest way to do it?
If you use the cos(A+B) rule and split up the 75 into terms that you can workout.

So cos75 = cos(30 + 45)

= cos30cos45 - sin30sin45

cos 30 = (sqrt3)/2
sin 45 = cos 45 = 1/(sqrt 2)
sin 30 = 1/2

sub all these values into the equation and voila!!! There's you ans in surd form

Hope that makes sense

Edit: Mockel and Mysticmin beat me to it!!!
5. Damn, mockel beat me to it

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Updated: June 19, 2004
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