trig question!!

[uki323]

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ugh, i hate trig!! 🙁

anyways, I was wondering if anyone can explain this:
sin^2@ - cos^2@ = - cos 2@

@= theta lol

this problem is from achiever, the 2nd set of QR...




Thanks a lot in advance!! 🙂

 

[nze82]

ugh, i hate trig!! 🙁

anyways, I was wondering if anyone can explain this:
sin^2@ - cos^2@ = - cos 2@

@= theta lol

this problem is from achiever, the 2nd set of QR...




Thanks a lot in advance!! 🙂

You just need to know the formula:
Cos(2x) = Cos^2(x) - Sin^2(x)
So:
Sin^2(x) - Cos^2(x) = - Cos(2x)

 

[iwannabadentist]

this is a trig identity:
cos2@ = cos^2@ - sin^2@

if u multiply cos2@ by -1, you must multiply the other side of the equation by -1 also, so you would have:
-cos^2@ - (-sin^2@) = sin^2@ - cos^2@

you need to memorize some basic trig identities

http://www.coolschool.ca/lor/PMA12/unit5/U05L06.htm

 

[jsa86]

cos2 = cos^2 - sin^2

so, if you just move things around you can get the answer your looking for. that formula is just one to memorize

 

[uki323]

wow, thank you so much!! 😀