Asked by Lindsay
rewrite using radicals and rational numbers
Cos 3(pi)/8
Cos 3(pi)/8
Answers
Answered by
drwls
Let a = 3 pi/4 = 135 degrees
cos a = -(sqrt 2)/2
You want cos (a/2)= cos (3 pi/8)
Use the identity:
cos (a/2) = sqrt[(1/2)(1 + cos a)]
= sqrt[(1/2)(1 - (sqrt2/2)]
= sqrt[(1/2) -(sqrt2)/4]
cos a = -(sqrt 2)/2
You want cos (a/2)= cos (3 pi/8)
Use the identity:
cos (a/2) = sqrt[(1/2)(1 + cos a)]
= sqrt[(1/2)(1 - (sqrt2/2)]
= sqrt[(1/2) -(sqrt2)/4]
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