Can we conclude that the equation cos (a-b)= cos a-cos B is an identity?
回答 (3)
2016-08-25 6:50 pm
No.
cos (a - b) = cos a cos b + sin a sin b
cos (a - b) = cos a cos b + sin a sin b
2016-08-25 6:36 pm
No
cos (a-b)= cos a cos b - sin a sin b
cos (a-b)= cos a cos b - sin a sin b
2016-08-25 6:03 pm
Let's check for a= pi/2 and b = pi/4
cos(pi/2 - pi/4) = cos(pi/2) - cos(pi/4)
cos(pi/4) = cos(pi/2) - cos(pi/4)
sqrt(2)/2 = 0 - sqrt(2)/2
sqrt(2)/2 = -sqrt(2)/2
This does not work, so no... this is not an identity.
cos(pi/2 - pi/4) = cos(pi/2) - cos(pi/4)
cos(pi/4) = cos(pi/2) - cos(pi/4)
sqrt(2)/2 = 0 - sqrt(2)/2
sqrt(2)/2 = -sqrt(2)/2
This does not work, so no... this is not an identity.
收錄日期: 2021-04-21 21:21:32
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20160825083747AAyONuH