Find the exact value of tan 15 degrees

tan(15) = 
sin(15) / cos(15) = 
sin(45 - 30) / cos(45 - 30) = 
[ sin(45)cos(30) - sin(30)cos(45) ] / [ cos(45)cos(30) + sin(45)sin(30)] 

Since sin(45) = cos(45) = √2/2, you can just factor that out from the top and bottom 
[ cos(30) - sin(30) ] / [ cos(30) + sin(30)] 
[ √3/2 - 1/2 ] / [ √3/2 + 1/2] 
(√3 - 1) / (√3 + 1) 
(√3 - 1)^2 / (√3+1)(√3 - 1) 
(√3 - 1)^2 / (3 - 1) 
(3 - 2√3 +1) / 2 
2 - √3 

There's also a formula for tan(a-b), but I couldn't remember it off hand.

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