a.maths黎
cos2x=sin^4 x +cos^4 x
ans
0,180,360
求步驟
數學高手啊！
2010-01-16 9:04 pm
回答 (1)
2010-01-16 9:21 pm
✔ 最佳答案
cos2x=sin^4 x +cos^4 x2cos^2x - 1=( sin^2 x +cos^2 x)^2 - 2cos^2xsin^2x
2cos^2x - 1= 1 - 2cos^2xsin^2x
2cos^2x - 1 + 2cos^2xsin^2x - 1= 0
2cos^2x - 2 +2cos^2x(1-cos^2x) = 0
4cos^2x -2 -2cos^4x = 0
cos^4x - 2cos^2x +1 = 0
(cos^2x - 1)^2 = 0
cos^2x = 1
cosx = 1 or cosx =-1
x = 0,180,360
收錄日期: 2021-04-13 17:02:46
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20100116000051KK00588