Posted by Jack on October 21, 2002 at 20:02:52:
In Reply to: pre-calc-is this the right way???? posted by Jacquie on October 21, 2002 at 18:17:29:
: find exact value of cos 11pi/12
: i used the double-angle formula for cos
: sq. root of (1 + cos 11pi/12)/2
: sq. root of(1+cos(8pi/12 + 3pi/12))/2
: sq. root of(1+cos(2pi/3 + pi/4))/2
: sq. root of(1+(-1/2)+ (sq.root of2/2))/2
: sq. root of(2/2+sq. root of2-1/2/2
: sq.root of(sq. root of 2 +1/2)/2
: but i dont know how to further reduce this???
: You are using the half-angle identity
: 11pi/12 = 165 degrees
: cos(165) = cos(330/2) = sqrt[(1+cos(330))/2]
: = sqrt[(1+cos(30))/2]
: = -.966