Trig question

[RandyJ 10+ year member]

Pain in my *** studying for a test

Anywhoo, simplify this, without the use of a calculator. I can tell you what you should get, tell me how I get it please.

cos(2sqrt5/5 + sqrt5/3) = cos(2sqrt5/5)cos(sqrt5/3)-sin(2sqrt5/5)sin(sqrt5/3)

book gives (10-2sqrt5)/15

I see where all the figures are coming from, but cannot put it all together.

Click to read more...

 

[Beat_Dominator 5,000+ posts]

Just assign "2sqrt5/5" to equal X ; and "sqrt5/3" to be Y.

you'll have Cos(X+Y)=cosXcosY-sinXsinY....

 

[RandyJ 10+ year member]

Just assign "2sqrt5/5" to equal X ; and "sqrt5/3" to be Y.
you'll have Cos(X+Y)=cosXcosY-sinXsinY....

//content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif

Going from

cosXcosY-sinXsinY to (10-2sqrt5)/15 is the problem I am having. I understand the identity itself, just not the math following.

 

[Beat_Dominator 5,000+ posts]

Sheesh.

 

[RandyJ 10+ year member]

Sheesh.

Is there something wrong? Am I making a foolish error and not catching the obvious?

 

[Beat_Dominator 5,000+ posts]

I dunno, lemme look.

 

[Beat_Dominator 5,000+ posts]

Eh I'm not getting it either //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif My day has been too long for trig identities //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif

Tell me if you get the answer or I'll have to solve it later because it'll bother me //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif